System and method for generating and analyzing roughness measurements and their use for process monitoring and control

ABSTRACT

In one embodiment, a method includes receiving measured linescan information describing a pattern structure of a feature, applying the received measured linescan information to an inverse linescan model that relates measured linescan information to feature geometry information, and identifying, based at least in part on the applying the received measured linescan model to the inverse linescan model, feature geometry information that describes a feature that would produce a linescan corresponding to the received measured linescan information. The method also includes determining, at least in part using the inverse linescan model, feature edge positions of the identified feature, analyzing the feature edge positions to determine errors in the manufacture of the pattern structure, and controlling a lithography tool based on the analysis of the feature edge positions.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation-in-part of U.S. applicationSer. No. 16/730,393 filed Dec. 20, 2019 titled “System and Method forGenerating and Analyzing Roughness Measurements”, which is acontinuation of U.S. application Ser. No. 16/218,346 titled “System andMethod for Generating and Analyzing Roughness Measurements” (now U.S.Pat. No. 10,522,322), which is a continuation-in-part of and claimspriority to U.S. application Ser. No. 15/892,080 filed Feb. 8, 2018titled “Edge Detection System” (now U.S. Pat. No. 10,176,966). Thisapplication further claims priority to U.S. Provisional PatentApplication No. 62/739,721 filed Oct. 1, 2018 titled “System and Methodfor Generating and Analyzing Roughness Measurements” and U.S.Provisional Patent Application No. 62/678,866 filed May 31, 2018 titled“System and Method for Removing Noise From Roughness Measurements.”Further, as a continuation of U.S. application Ser. No. 16/218,346, thisapplication claims priority to U.S. Provisional Patent Application Ser.No. 62/602,152, filed Apr. 13, 2017 and titled “Edge Detection System.”All applications are incorporated by reference herein as if reproducedin full below.

This patent application is a continuation of U.S. application Ser. No.16/716,131 filed Dec. 16, 2019 titled “Edge Detection System”, which isa continuation of U.S. application Ser. No. 16/222,668 filed Dec. 17,2018 titled “Edge Detection System,” (now U.S. Pat. No. 10,510,509),which is a continuation of U.S. application Ser. No. 15/892,080 filedFeb. 8, 2018 titled “Edge Detection System” (now U.S. Pat. No.10,176,966). The Ser. No. 15/892,080 application claims priority to U.S.Provisional Patent Application Ser. No. 62/602,152, filed Apr. 13, 2017and also titled “Edge Detection System.” Both applications areincorporated by reference herein as if reproduced in full below.

BACKGROUND

The disclosures herein relate generally to roughness measurements ofpattern structures, and more particularly, to roughness measurements ofpattern structures in noise-prone images, such as in images formed whenusing a scanning electron microscope (SEM) or other imaging apparatusthat produce images including undesired noise, and even moreparticularly, to analyzing such roughness measurements to removeunwanted artifacts (spikes) and measure desired features (bumps).Further, the disclosures generally relate to controlling a manufacturingprocess through the use of such measurements.

BRIEF SUMMARY

Disclosed herein are systems and methods that remove noise fromroughness measurements to determine roughness of a feature in a patternstructure and extracting useful information from those measurements. Inone embodiment, a method for determining roughness of a feature in apattern structure includes generating, using an imaging device, a set ofone or more images. Each image of the set includes one or more instancesof a feature within a respective pattern structure, and each imageincludes measured linescan information corresponding to the patternstructure that includes noise. The method also includes detecting edgesof the features within the pattern structure of each image of the setwithout filtering the images, generating a biased power spectral density(PSD) dataset representing feature geometry information corresponding tothe edge detection measurements of the set of images, evaluating ahigh-frequency portion of the biased PSD dataset to determine a noisemodel for predicting noise over all frequencies of the biased PSDdataset, and subtracting the noise predicted by the determined noisemodel from a biased roughness measure to obtain an unbiased roughnessmeasure. The method further includes filtering out unwanted spikes inthe PSD caused by measurement artifacts or other phenomenon to betterreveal the true PSD behavior of the rough features. The method furtherincludes modeling the PSD to extract information about bumps in the PSDcaused by, for example, mask roughness or stress-related wiggle.

The method may further includes using the unbiased roughness measure toassess the quality of the pattern structure. The method further includesusing the unbiased roughness measure to assess the quality of theprocess and process materials used to fabricate the pattern structure.The method further includes using the unbiased roughness measure toassess the quality of the process equipment used to fabricate thepattern structure. The method further includes using the biased andunbiased roughness measures to assess the quality of the metrology usedto measure the pattern structure. The method further includes using theunbiased roughness measure and said quality assessment to make changesto the fabrication process, fabrication process equipment, or metrologyprocess or equipment to affect an improvement in quality of thepatterned structure, either in a manual or automated way. The methodfurther includes using the unbiased roughness measure and said qualityassessment to make changes to the fabrication process, fabricationprocess equipment, or metrology process or equipment to affect anincrease in throughput of the fabrication process without loss ofquality of the patterned structure. The method further includes usingthe unbiased roughness measure and said quality assessment to makechanges to the fabrication process, fabrication process equipment, ormetrology process or equipment to affect a reduction in cost of thefabrication process without loss of quality of the patterned structure.The method further includes an automated feedback system to perform suchprocess changes.

In another embodiment, a system for determining roughness of a featurein a pattern structure includes an imaging device for generating a setof one or more images, and a processor. Each image of the set includesone or more instances of a feature within a respective patternstructure, and each image includes measured linescan informationcorresponding to the pattern structure that includes noise. Theprocessor is coupled to receive the measured linescan information fromthe imaging device. The processor is configured to detect edges of thefeatures within the pattern structure of each image of the set withoutfiltering the images, generate a biased power spectral density (PSD)dataset representing feature geometry information corresponding to theedge detection measurements of the set of images, evaluate ahigh-frequency portion of the biased PSD dataset to determine a noisemodel for predicting noise over all frequencies of the biased PSDdataset, subtract the noise predicted by the determined noise model froma biased roughness measure to obtain an unbiased roughness measure, andfilter and/or model the PSD to remove unwanted artifacts and revealartifacts of interest. In some embodiments, the processor is configuredto use the obtained roughness measures to assess the quality of thepatterned structure and its fabrication process, and control thefabrication process based on such roughness measures and qualityassessments.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended drawings illustrate only exemplary embodiments of theinvention and therefore do not limit its scope because the inventiveconcepts lend themselves to other equally effective embodiments.

FIG. 1A is a representation of a pattern structure that exhibitsparallel line features with spaces in between the lines.

FIG. 1B is a representation of a pattern structure that includes contacthole features.

FIG. 2 shows four different rough edges, all with the same standarddeviation.

FIG. 3 is a representation of power spectral density (PSD) vs. frequencyon a log-log scale.

FIG. 4 is a graphic representation of power spectral density (PSD)plotted vs. frequency and depicting roughness parameters PSD(0),correlation length, and roughness exponent.

FIG. 5 shows two power spectral densities (PSDs) corresponding torespective edges of a feature on a pattern structure.

FIG. 6 is a graphic representation of the tradeoff of within-featurevariation and feature-to-feature variation as a function of line length.

FIG. 7 is a block diagram of a scanning electron microscope (SEM)coupled to an information handling system (IHS) that together form oneembodiment of the disclosed edge detection apparatus.

FIG. 8A is a representation of a feature disposed on a substrate thatdepicts an electron beam impinging on the center of the feature.

FIG. 8B is a representation of a feature disposed on a substrate thatdepicts an electron beam impinging on the feature near its edge.

FIG. 9 shows a gray scale image representation on top with acorresponding grayscale linescan along one horizontal cut beinggraphically plotted immediately below.

FIG. 10 shows an example of a pattern structure including a featuresituated atop a substrate with varying numbers of electrons escapingfrom the pattern structure depending on where the electron beam impingeson the pattern structure.

FIG. 11 shows a predicted linescan of a resist step on a patternstructure such as a silicon wafer.

FIG. 12 shows another representative predicted linescan of a pattern ofresist lines and spaces on a silicon wafer.

FIG. 13A is an original SEM image of a pattern structure without usingthe disclosed edge detection apparatus and method.

FIG. 13B is the same SEM image as FIG. 13A except using the disclosededge detection apparatus and method.

FIG. 14 is a Raw (Biased) linewidth roughness plot vs. thresholdsettings showing both a prior art result (using a filter withconventional threshold edge detection), and a result using no filter andan inverse linescan model (ILM).

FIG. 15A is a power spectral density (PSD) vs. frequency plot of theright and left edges of a feature shown before noise subtraction.

FIG. 15B is a power spectral density (PSD) vs. frequency plot of theright and left edges of a feature shown after noise subtraction.

FIG. 16 shows portions of three SEM images of nominally the samelithographic features taken at different SEM electron doses.

FIG. 17A shows a typical linescan for a line feature on a wafer for acase when there is an extremely large number of electrons so that thepixel noise is negligible.

FIG. 17B shows the 1-sigma uncertainty in edge detection position forperfectly smooth features in the presence of grayscale noise, for threedifferent X pixel sizes.

FIG. 17C shows grayscale images as an example of using a simplethreshold edge detection algorithm with image filtering in the rightimage, and without image filtering in the left image.

FIG. 18 is a plot of linewidth roughness (LWR) PSD vs. frequency thatshows the impact of two different image filters on a collection of 30images.

FIG. 19 is a power spectral density plot vs. frequency that shows thenoise subtraction process of the disclosed edge detection apparatus andmethod.

FIG. 20 shows PSDs of a particular resist feature type on a given wafer,measured with different frames of integration in the SEM.

FIG. 21 shows the biased and unbiased values of the 36 linewidthroughness (LWR) measured as a function of the number of frames ofintegration in the SEM.

FIG. 22A shows biased linewidth roughness (LWR) power spectral densities(PSDs) as a function of different pixel sizes and magnificationsemployed by the SEM.

FIG. 22B shows unbiased linewidth roughness (LWR) power spectraldensities (PSDs) as a function of different pixel sizes andmagnifications employed by the SEM.

FIG. 23 is a flowchart that depicts a representative overall processflow that the disclosed SEM edge detection system employs to detectedges of a pattern structure.

FIG. 24A is a grayscale representation of a pattern structure ofvertical lines and spaces that the disclosed metrology tool analyzes.

FIG. 24B shows a single linescan at one Y-pixel position.

FIG. 24C shows the averaged linescan that is generated by averaging overall Y-pixels.

FIG. 25A shows a PSD that includes high-frequency spike artifacts.

FIG. 25B shows the PSD with spike artifacts removed.

FIG. 26 shows a PSD that includes mid-frequency spike artifacts andharmonics.

FIG. 27A shows the impact of mid-frequency spike artifacts on themodeling and interpreting of the PSD.

FIG. 27B shows the impact of removing mid-frequency spike artifacts onthe modeling and interpreting of the PSD.

FIG. 28A shows a PSD dataset that exhibits a type of bump behavior.

FIG. 28B shows an additional PSD dataset that exhibits a type of bumpbehavior.

FIG. 29A shows the modeling and analysis of a low frequency bump of typeI.

FIG. 29B shows the modeling and analysis of a low frequency bump of typeII.

FIG. 30 is a flowchart that depicts a representative process flow todetect undesired spikes in a PSD dataset, and to remove the spikes fromthe PSD dataset and obtain roughness parameters for a feature.

FIG. 31 is a flowchart that depicts another representative process flowto model bumps in a PSD dataset, and to obtain unbiased roughnessparameters for a feature.

DETAILED DESCRIPTION

Measuring the roughness of a pattern is complicated by that fact thatnoise in the measurement system is difficult to differentiate from theroughness being measured. It is common to using an imaging tool, such asa microscope, to create a detailed image of an object to be measured andthen analyze the information on that image to measure and characterizethe roughness of one or more features of the object. In this case, noisein the acquired image can appear to be roughness of the features in theimage. Described below are techniques useful to separate the noise inthe image from the actual roughness of the features in order to producemore accurate measurements of the roughness of the features.

Measuring the roughness of a pattern is further complicated by that factthat undesirable artifacts at specific spatial frequencies can bepresent in the images. These artifacts might be caused by imaginganomalies such as jitter in the scanning used to acquire the image.These artifacts might also be caused by physical aspects of the objectto be measured, such as regular topographical structures lying below thefeatures to be measured, that interfere with the measurement of thosefeatures.

As an example, scanning electron microscopes (SEMs) are very useful forstudying the features of pattern structures, such as semiconductordevices, for example. Unfortunately, measuring feature roughness ofthese structures is often challenging because of the noise that isinherent in SEM images. Filtering (smoothing) of the SEM image istypically needed to achieve accurate edge detection, but such filteringundesirably changes the feature roughness that is measured. An edgedetection approach is needed that reliably detects edges in very noisySEM images without the use of image filtering (or at least without anyfiltering that would change the feature roughness that is measured).

Pattern roughness is a major problem in many fields. Many if not alltechniques for creating patterns of various shapes produce roughness onthe edges of those patterns, at least on the near molecular scale if notlarger scales. For example, in advanced lithography for semiconductormanufacturing, especially for extreme ultraviolet (EUV) lithography butfor other lithography methods as well, roughness of the printed andetched patterns can cause many negative effects. Reduction in roughnessrequires a better understanding of the sources of stochastic variation,which in turn requires better measurement and characterization of roughfeatures. Prior art roughness measurement approaches suffer from severebias because noise in the image adds to the roughness on the wafer. Thedisclosures herein provide a practical approach to making unbiasedroughness measurements through the use of a physics-based inverselinescan model. This enables accurate and robust measurement ofroughness parameters over a wide range of SEM metrology conditions.

Before discussing embodiments of the disclosed technology that addressthe SEM image noise problem, this disclosure first discusses lithographyof pattern structures and the frequency dependence of roughness.

1. Stochastic Effects in Lithography

Lithography and patterning advances continue to propel Moore's Law bycost-effectively shrinking the area of silicon consumed by a transistorin an integrated circuit. Besides the need for improved resolution,these lithography advances should also allow improved control of thesmaller features being manufactured. Historically, lithographers focusedon “global” sources of variation that affect patterning fidelity (e.g.,exposure dose and focus variations, hotplate temperature non-uniformity,scanner aberrations) by attempting to minimize the sources of thesevariations and by developing processes with minimum sensitivity to thesevariations. Today's small features, however, also suffer from “local”variations caused by the fundamental stochastics of patterning near themolecular scale.

In lithography, light is used to expose a photosensitive material calleda photoresist. The resulting chemical reactions (including those thatoccur during a post-exposure bake) change the solubility of the resist,enabling patterns to be developed and producing the desired criticaldimension (CD). For a volume of resist that is “large” (that is, avolume that contains many, many resist molecules), the amount of lightenergy averaged over that volume produces a certain amount of chemicalchange (on average) which produces a certain (average) amount ofdissolution to create the pattern. The relationships between lightenergy, chemical concentration, and dissolution rate can be describedwith deterministic equations that predict outputs for a given set ofinputs. These models of lithography are extremely useful and arecommonly used to understand and control lithography processes forsemiconductor manufacturing.

This deterministic view of a lithography process (certain inputs alwaysproduce certain outputs) is only approximately true. The “mean fieldtheory” of lithography says that, on average, the deterministic modelsaccurately predict lithographic results. If we average over a largenumber of photons, a single number for light energy (the average) issufficient to describe the light energy. For a large volume of resist,the average concentration of a chemical species sufficiently describesits chemical state. But for very small volumes, the number of atoms ormolecules in the volume becomes random even for a fixed “average”concentration. This randomness within small volumes (that is, for smallquantities of photons or molecules or numbers of events) is generallyreferred to as “shot noise”, and is an example of a stochastic variationin lithography that occurs when the region of interest approaches themolecular scale.

A stochastic process is one in which the results of the process arerandomly determined. At the atomic/molecular level, essentially allprocesses are stochastic. For semiconductor patterning at the 20-nm nodeand below (with minimum feature sizes below 40 nm), the dimensions ofinterest are sufficiently small that stochastic effects become importantand may even dominate the total variations that affect the dimensions,shapes, and placements of the patterns being fabricated. Thesestochastic effects can also be important for larger feature sizes undersome circumstances.

The most prominent manifestation of stochastic variations in lithography(as well as etch and other parts of the patterning process) is that thepatterns being produced are rough rather than smooth (FIG. 1A). In thepattern structure shown in FIG. 1A, nominally parallel vertical linesappear as bright vertical regions, while spaces appear as dark verticalregions between the lines. The roughness of the edge of a feature iscalled line-edge roughness (LER), and the roughness of the width of afeature is called linewidth roughness (LWR). The roughness of thecenterline of the feature (the midpoint between left and right edges) iscalled pattern placement roughness (PPR). Another important consequenceof these stochastic variations is the random variation of the size,shape, and placement of features, which are especially evident forcontact hole features (FIG. 1B).

Stochastic effects in patterning can reduce the yield and performance ofsemiconductor devices in several ways: a) Within-feature roughness canaffect the electrical properties of a device, such as metal lineresistance and transistor gate leakage; b) Feature-to-feature sizevariation caused by stochastics (also called local CD uniformity, LCDU)adds to the total budget of CD variation, sometimes becoming thedominant source; c) Feature-to-feature pattern placement variationcaused by stochastics (also called local pattern placement error, LPPE)adds to the total budget of PPE, sometimes becoming the dominant source;d) Rare events leading to greater than expected occurrence ofcatastrophic bridges or breaks are more probable if error distributionshave fat tails; and e) Decisions based on metrology results (includingprocess monitoring and control, as well as the calibration of opticalproximity correction (OPC) models) can be poor if those metrologyresults do not properly take into account stochastic variations. Forthese reasons, proper measurement and characterization ofstochastic-induced roughness is critical.

Many other kinds of devices are also sensitive to feature roughness. Forexample, roughness along the edge of an optical waveguide can cause lossof light due to scattering. Feature roughness in radio frequencymicroelectromechanical systems (MEMS) switches can affect performanceand reliability, as is true for other MEMS devices. Feature roughnesscan degrade the output of light emitting diodes. Edge roughness can alsoaffect the mechanical and wetting properties of a feature inmicrofluidic devices. Roughness of the features in a wire grid polarizercan affect the efficiency and transmission of the polarizer.

Unfortunately, prior art roughness measurements (such as the measurementof linewidth roughness or line-edge roughness using a critical dimensionscanning electron microscope, CD-SEM) are contaminated by measurementnoise caused by the measurement tool. This results in a biasedmeasurement, where the measurement noise adds to the true roughness toproduce an apparent roughness that overestimates the true roughness.Furthermore, these biases are dependent on the specific measurement toolused and on its settings. These biases are also a function of thepatterns being measured. Prior art attempts at providing unbiasedroughness estimates often struggle in many of today's applications dueto the smaller feature sizes and higher levels of SEM noise.

Thus, there is a need for a new approach to making unbiased roughnessmeasurements that avoids the problems of prior art attempts and providesan unbiased estimate of the feature roughness that is both accurate andprecise. Further, a good pattern roughness measurement method shouldhave minimum dependence on metrology tool settings. CD-SEM settings suchas magnification, pixel size, number of frames of averaging (equivalentto total electron dose in the SEM), voltage, and current may causefairly large changes in the biased roughness that is measured. Ideally,an unbiased roughness measurement would be independent of these settingsto a large degree.

Additionally, bias in the measurement of roughness degrades the qualityand usefulness of the roughness measurements themselves. The results ofroughness measurement can be used in many ways to make many types ofdecisions, for example, in an integrated circuit manufacturing process.A measurement of pattern structure roughness can be used to assess thequality of the device being fabricated, and to predict yield andperformance of the devices being fabricated. If the predicted yield orperformance is sufficiently poor, a decision could be made to stopfurther processing of those devices and scrap the specific devices, theintegrated circuit in which the devices were found, the wafer orsubstrate on which the integrated circuit was found, or themanufacturing lot or batch in which the wafer was found.

A further use for the results of a measurement of pattern roughness isin the assessment of the quality of the manufacturing process ormanufacturing tools that were used to make the measured patterns. Suchan assessment could be used to affect changes in the manufacturingprocess or tool to improve pattern quality. For example, patternroughness is sensitive to the focus setting of a lithography tool usedto print the patterns. An increase in measured pattern roughness couldbe used to trigger a focus check and focus adjustment of the lithographytool, thus improving the quality of subsequently printed patterns.Degradation in the quality of the roughness measurements could result indegradation in the quality of the process decisions made using thosemeasurements, such as making a focus change when none is needed, or notmaking a focus change when one is needed.

The use of unbiased measurements of roughness would improve the qualityof the decisions made, for example, the decision to scrap a wafer orlot, or the decision to change a process or tool setting. Since the biasin a roughness measurement can change from measurement to measurementeven if the true roughness of the pattern structure does not change, theuse of biased roughness measurements for these and other decisions isproblematic.

A further use for the results of a measurement of pattern roughness isin the assessment of the quality of the metrology used to make themeasurements. If, for example, a change in the bias in the measurementswere detected, adjustment to the measurement tool (for example, the SEM)could be made to improve the quality and reliability of themeasurements.

2. The Frequency Dependence of Line-Edge Roughness (LER), Line-WidthRoughness (LWR), and Pattern Placement Roughness (PPR)

Rough features are most commonly characterized by the standard deviationof the edge position (for LER), linewidth (for LWR), or featurecenterline (for PPR). But describing the standard deviation is notenough to fully describe the roughness. FIG. 2 shows four differentrough edges, all with the same standard deviation. The prominentdifferences visible in the edges make it clear that the standarddeviation is not enough to fully characterize the roughness. Instead, afrequency analysis of the roughness is required. The four randomly roughedges depicted in FIG. 2 all have the same standard deviation ofroughness, but differ in the frequency parameters of correlation length(ξ) and roughness exponent (H). More specifically, with respect to FIG.2, in case a) ξ=10, H=0.5; in case b) ξ=10, H=1.0; in case c) ξ=100,H=0.5; and in case d) ξ=0.1, H=0.5.

The standard deviation of a rough edge describes its variation relativeto and perpendicular to an ideal straight line. In FIG. 2, the standarddeviation describes the vertical variation of the edge. But thevariation can be spread out differently along the length of the line (inthe horizontal direction in FIG. 2). This line-length dependence can bedescribed using a correlation function such as the autocorrelationfunction or the height-height correlation function.

Alternatively, the frequency f can be defined as one over a length alongthe line (FIG. 3). The dependency of the roughness on frequency can becharacterized using the well-known power spectral density (PSD). The PSDis the variance of the edge per unit frequency (FIG. 3), and iscalculated as the square of the coefficients of the Fourier transform ofthe edge deviation. The low-frequency region of the PSD curve describesedge deviations that occur over long length scales, whereas thehigh-frequency region describes edge deviations over short lengthscales. Commonly, PSDs are plotted on a log-log scale as used in FIG. 3.

The PSD of lithographically defined features generally has a shapesimilar to that shown in FIG. 3. The low-frequency region of the PSD isflat (so-called “white noise” behavior), and then above a certainfrequency it falls off as a power of the frequency (a statisticallyfractal behavior). The difference in these two regions has to do withcorrelations along the length of the feature. Points along the edge thatare far apart are uncorrelated with each other (statisticallyindependent), and uncorrelated noise has a flat power spectral density.But at short length scales the edge deviations become correlated,reflecting a correlating mechanism in the generation of the roughness,such as acid reaction-diffusion for a chemically amplified resist. Thetransition between uncorrelated and correlated behavior occurs at adistance called the correlation length.

FIG. 4 shows that a typical PSD curve can be described with threeparameters. PSD(0) is the zero-frequency value of the PSD. While thisvalue of the PSD can never be directly measured (zero frequencycorresponds to an infinitely long line), PSD(0) can be thought of as thevalue of the PSD in the flat low-frequency region. The PSD begins tofall near a frequency of 1/(2πξ) where j is the correlation length. Inthe fractal region, we have what is sometimes called “1/f” noise and thePSD has a slope (on the log-log plot) corresponding to a power of 1/f.The slope is defined as 2H+1 where H is called the roughness exponent(or Hurst exponent). Typical values of H are between 0.5 and 1.0. Forexample, H=0.5 when a simple diffusion process causes the correlation.Each of the parameters of the PSD curve has important physical meaningfor a lithographically defined feature as discussed in more detailbelow. The variance of the roughness is the area under the PSD curve andcan be derived from the other three PSD parameters. The exactrelationship between variance and the other three PSD parameters dependson the exact shape of the PSD curve in the mid-frequency region (definedby the correlation length), but an approximate relationship can be usedto show the general trend, as per EQUATION 1 below:

$\begin{matrix}{\sigma^{2} \approx \frac{{PSD}(0)}{\left( {{2H} + 1} \right)\xi}} & {{EQUATION}\mspace{14mu} 1}\end{matrix}$

The differences observed in the respective four rough edges of FIG. 2can now be easily seen as differences in the PSD behavior of thefeatures. FIG. 5 shows two PSDs, corresponding to edge a) and edge c)from FIG. 2. While these two edges have the same variance (the same areaunder the PSD curve), they have different values of PSD(0) andcorrelation length (in this case the roughness exponent was keptconstant). Although the standard deviations of the roughness of edge a)and edge c) are the same, these edges exhibit different PSD behaviors.As discussed below, the different PSD curves will result in differentroughness behavior for lithographic features of finite length.

3. Impact of the Frequency Behavior of Roughness

The roughness of the lines and spaces of pattern structures ischaracterized by measuring very long lines and spaces, sufficiently longthat the flat region of the PSD becomes apparent. For a sufficientlylong feature the measured LWR (that is, the standard deviation 6 of themeasured linewidths along the line) can be thought of as the LWR of aninfinitely long feature, σ_(LWR)(∞). But pattern structures such assemiconductor devices are made from features that have a variety oflengths L. For these shorter features, stochastics will causewithin-feature roughness, σ_(LWR)(L), and feature-to-feature variationdescribed by the standard deviation of the mean linewidths of thefeatures, σ_(CDU)(L). This feature-to-feature variation is called thelocal critical dimension uniformity, LCDU, since it represents CD(critical dimension) variation that is not caused by the well-known“global” sources of error (scanner aberrations, mask illuminationnon-uniformity, hotplate temperature variation, etc.).

For a line of length L, the within-feature variation and thefeature-to-feature variation can be related to the LWR of an infinitelylong line (of the same nominal CD and pitch) by the Conservation ofRoughness principle given in EQUATION 2 below:σ_(DU) ²(L)+σ_(LWR) ²(L)=σ_(LWR) ²(∞)  EQUATION 2

The Conservation of Roughness principle says that the variance of a verylong line is partitioned for a shorter line into within-featurevariation and feature-to-feature variation. How this partition occurs isdetermined by the correlation length, or more specifically by U. Using abasic model for the shape of the PSD as an example, it is seen that:

$\begin{matrix}{{\sigma_{CDU}^{2}(L)} = {\frac{{PSD}(0)}{L}\left\lbrack {1 - {\frac{\xi}{L}\left( {1 - e^{{- L}/\xi}} \right)}} \right\rbrack}} & {{EQUATION}\mspace{14mu} 3}\end{matrix}$

Thus, EQUATIONS 1-3 show that a measurement of the PSD for a long line,and its description by the parameters PSD(0), ξ, and H, enables one topredict the stochastic influence on a line of any length L. It is notedthat the LCDU does not depend on the roughness exponent, making H lessimportant than PSD(0) and ξ. For this reason, it useful to describe thefrequency dependence of roughness using an alternate triplet ofparameters: σ_(LWR)(∞), PSD(0), and ξ. Note that these samerelationships apply to LER and PPR as well.

It is also noted that, examining EQUATION 3, the correlation length isthe length scale that determines whether a line of length L acts “long”or “short”. For a long line, L>>ξ and the local CDU behaves as perEQUATION 4 below:

$\begin{matrix}{{{\sigma_{CDU}(L)} \approx \sqrt{\frac{{PSD}(0)}{L}}}{{{when}\mspace{14mu} L}\operatorname{>>}\xi}} & {{EQUATION}\mspace{14mu} 4}\end{matrix}$

This long-line result provides a useful interpretation for PSD(0): It isthe square of the LCDU for a given line times the length of that line.Reducing PSD(0) by a factor of 4 reduces the LCDU by a factor of 2, andthe other PSD parameters have no impact (so long as L>>ξ). Typically,resists have yielded correlation lengths on the order of one quarter toone half of the minimum half-pitch of their lithographic generation.Thus, when features are longer than approximately five times the minimumhalf-pitch of the technology node, we are generally in this long linelength regime. For shorter line lengths, the correlation length beginsto matter as well.

EQUATIONS 1-3 show a trade-off of within-feature variation andfeature-to-feature variation as a function of line length. FIG. 6 showsan example of this relationship. For very long lines, LCDU is small andwithin-feature roughness approaches its maximum value. For very shortlines the LCDU dominates. However, due to the quadratic nature of theConservation of Roughness, σ_(LWR)(L) rises very quickly as L increases,but LCDU falls very slowly as L increases. Thus, there is a wide rangeof line lengths where both feature roughness and LCDU are significant.

Since the Conservation of Roughness principle applies to PPR as well,short features suffer not only from local CDU problems but also fromlocal pattern placement errors (LPPE) as well. For the case ofuncorrelated left and right edges of a feature, the PSD(0) for LWR istypically twice the PSD(0) of the LER. Likewise, the PSD(0) of the LERis typically twice the PSD(0) of the PPR. Thus, in general, the LPPE isabout half the LCDU. When left and right feature edges are significantlycorrelated, these simple relationships no longer hold.

The above equations allow a measurement of a very long pattern structure(where measurements are approaching the value one would obtain for aninfinitely long feature) to predict the within-feature andfeature-to-feature variation of shorter features of any length. Forexample, the feature length for such a prediction can be chosen to matchthe feature length of device features of interest. Predictions of thevariations of this shorter feature could then be used to predict theyield and/or performance of a device that used such a feature usingwell-known models of device performance. Such models include, forexample, TCAD models that predict the electrical behavior of a singletransistor, or circuit timing models that predict the timing and skew ofan entire circuit or sub-circuit.

4. Measurements of the Roughness of Pattern Structures with a ScanningElectron Microscope (SEM)

A common way to measure feature roughness for small features is thetop-down critical dimension scanning electron microscope (CD-SEM).Typical light microscopes have magnifications up to 1000× andresolutions down to a few hundred nanometers. Scanning electronmicroscopes use electrons to create very small spots (near 1 nm inwidth) that can be used to create high-resolution images, withmagnifications above 20,000×. CD-SEMs are SEMs that have been optimizedfor measuring the dimensions of a wide range of features found onsemiconductor wafers. They can measure the mean critical dimension of arough feature with high precision, but have also proven very useful formeasuring LER, LWR, PPR, and their PSDs as well. However, there areerrors in the SEM images that can have large impacts on the measuredroughness and the roughness PSD while having little impact on themeasurement of mean CD. For this reason, the metrology approach neededfor PSD measurement may be quite different than the approach commonlyused for mean CD measurement.

FIG. 7 shows a block diagram of one embodiment of the disclosedmeasurement system 700 that determines feature roughness. The patternstructure sample 800 and the electron imaging optics (710, 715, 720,725) are situated in a vacuum chamber 703 that is evacuated by vacuumpump 702. Electrons are generated from a source such as an electron gun705 to form an electron beam 707. Common electron beam sources include aheated tungsten filament, a lanthanum hexaboride (LaB6) crystal formedinto a thermionic emission gun, or a sharp-tipped metal wire formed tomake a field emission gun. The emitted electrons are accelerated andfocused using electromagnetic condenser lenses 710, 715, and 720. Theenergy of the electrons striking the pattern structure sample 800 isgenerally in the 200 eV to 40 keV range in SEMs, but more typically 300eV to 800 eV for CD-SEMs. Final condenser lens 720 employs scanningcoils 725 to provide an electric field that deflects electron beam 707toward pattern structure 800 as a focused spot. Scanning coils 725 scanthe focused spot across the pattern structure 800 through final lensaperture 735 in a raster scan fashion to expose a specific field of viewon the pattern structure 800. SEM 701 includes a backscatter electrondetector 740 that detects backscatter electrons scattering back frompattern structure sample 800. SEM 700 also includes a secondary electrondetector 745, as shown in FIG. 7. Prior to imaging pattern structure800, the user places pattern structure 800 on a pattern structurereceiver 732 that supports and positions pattern structure 800 withinSEM 700. SEM 700 includes a controller (not shown) that controls theraster scanning of pattern structure 800 during imaging.

Referring now to FIGS. 8A and 8B, the electrons of electron beam 705that strike pattern structure sample 800 undergo a number of processesthat depend on the energy of the electron and the material properties ofthe sample. Electrons scatter off the atoms of the sample material,release energy, change direction, and often generate a cascade ofsecondary electrons by ionizing the sample atoms. Some of thesesecondary electrons may escape from the pattern structure (805) andothers may remain inside the pattern structure. Pattern structure 800includes a substrate 810, such as a semiconductor wafer. A feature 815is disposed atop substrate 810, as shown in FIG. 8A. Feature 815 may bea metallic line, a semiconductor line, a photoresist line or otherstructures on substrate 810. Feature 815 may have other shapes such as apillar or a hole, or more complicated shapes. Feature 815 may berepeating or isolated with respect to other features on the patternstructure. The space surrounding feature 815 may be empty (vacuum orair) or may be filled with a different material. Pattern structure 800may be a liquid crystal or other flat panel display, or other patternsemiconductor or non-semiconductor device. Feature 815 includes edges815-1 and 815-2. The region of feature 815 where electron beam 705interacts with feature 815 is the interaction volume 820 that exhibits,for example, a tear-droplet-like shape as depicted in FIG. 8A.

Occasionally electrons ricochet backwards off the atom nucleus and exitout of the sample (called backscatter electrons). Some of the lowerenergy secondary electrons can also escape out of the sample 805(frequently through the edges of a feature, see FIG. 8B). The way inwhich a SEM forms an image is by detecting the number of secondaryelectrons and/or backscatter electrons that escape the sample for eachbeam position.

As the electron beam is scanned across pattern structure sample 800during one linescan, it “dwells” at a specific spot for a specific time.During that dwell time, the number of electrons detected by either thebackscatter detector 725 or secondary electron detector 740, or both, isrecorded. The spot is then moved to the next “pixel” location, and theprocess is repeated. The result is a two-dimensional array of pixels(locations along the surface of the sample) with detected electroncounts digitally recorded for each pixel. The counts are typically thennormalized and expressed as an 8-bit grayscale value between 0 and 255.This allows the detected electron counts to be plotted as a grayscale“image”, such as those images shown in FIG. 1. While the image comingfrom a SEM reminds a viewer of an optical image as perceived through theeye, it is important to note that these grayscale images are actuallyjust convenient plots of the collected data.

A CD-SEM measures the width of a feature using the SEM image. The firststep in measuring feature width is to detect the edges of the features.For pixels near an edge of a feature, higher numbers of secondaryelectrons escape through the feature edge, producing bright pixelscalled “edge bloom” (see FIG. 8B and FIG. 9). It is this bright edgebloom that allows the feature edge to be detected. For example, in thegrayscale image representation in the upper portion of FIG. 9, such edgeblooms are observed at edges 905 and 910 of feature 915. A linescan isessentially a horizontal cut through a 2D SEM image that provides agrayscale value as a function of horizontal pixel position on thefeature, as in the graph shown in the bottom half of FIG. 9.

The data from a single horizontal row of pixels across the sample iscalled a “linescan”. Note that the term linescan is used here broadlyenough to include cases where an image is formed without the use ofscanning. The positions of the edges of a feature can be detected from asingle linescan, or from a collection of linescans representing theentire image, such as shown in the upper portion of FIG. 9. These sameedges appear as peaks 905′ and 910′ in the grayscale value vs. pixelposition graph in the lower portion of FIG. 9. Once the edges of aparticular feature have been determined, the width of the particularfeature is the difference between the positions of these two edges.

5. Linescan Models

Images are created through a physical process based on the microscope orother imaging tool used to acquire the image of a structure. Often theseimages are two-dimensional arrays of data, where the image can bethought of as a data set derived from the structure. A singleone-dimensional cut through the image is called a linescan. A model ofthe imaging tool can predict the image for a given structure beingimaged. For example, a model that describes a scanning electronmicroscope can predict the image that would be obtained by a SEM whenimaging a given structure.

A CD-SEM converts a measured linescan or a series of measured linescansinto a single dimension number, the measured CD. To better understandhow the linescan relates to the actual dimensions of the feature beingmeasured, it is important to understand how the systematic response ofthe SEM measurement tool to pattern structures impacts the shape of theresulting linescan. Rigorous 3D Monte Carlo simulations of SEM linescanscan be extremely valuable for this purpose, but they are often toocomputationally expensive for day-to-day use. Thus, one approach is todevelop a simplified analytical linescan model (ALM) that is morecomputationally appropriate to the task of quickly predicting linescans.The ALM employs the physics of electron scattering and secondaryelectron generation, and each term in the model has physicalsignificance. This analytical linescan expression can be fit to rigorousMonte Carlo simulations to both validate and calibrate its use.

The general application for the ALM has been the typical forwardmodeling problem: Given material properties (for the feature and thesubstrate) and a geometric description of the feature (width, pitch,sidewall angle, top corner rounding, footing, etc.), the ALM predictsthe linescan that would result. The mathematical details of the ALM arefound in the publications: Chris A. Mack and Benjamin D. Bunday,“Analytical Linescan Model for SEM Metrology”, Metrology, Inspection,and Process Control for Microlithography XXIX, Proc., SPIE Vol. 9424,94240F (2015), and Chris A. Mack and Benjamin D. Bunday, “Improvementsto the Analytical Linescan Model for SEM Metrology”, Metrology,Inspection, and Process Control for Microlithography XXX, Proc., SPIEVol. 9778, 97780A (2016), the disclosures of both publications beingincorporated herein by reference in their entireties. Other models withsimilar inputs and outputs can also be used.

The analytical linescan model (ALM) is briefly reviewed below. Themathematical modeling begins by assuming the interaction of the electronbeam with a flat sample of a given substance produces an energydeposition profile that takes the form of a double Gaussian, with aforward scattering width and a fraction of the energy forward scattered,and a backscatter width and a fraction of the energy deposited by thosebackscattered electrons. The model also assumes that the number ofsecondary electrons that is generated within the material is in directproportion to the energy deposited per unit volume, and the number ofsecondary electrons that escape the wafer (and so are detected by theSEM) are in direct proportion to the number of secondary electrons nearthe very top of the wafer.

The secondary electrons that reach the detector will emerge somedistance r away from the position of the incident beam. From theassumptions above, the number of secondary electrons detected will be afunction as given in EQUATION 5.f(r)=ae ^(−r) ² ^(/2σ) ^(f) ² +be ^(−r) ² ^(/2σ) ^(b) ²   EQUATION 5

where σ_(f) and σ_(b) are the forward and backscatter ranges,respectively, and a and b are the amounts of forward scattering andbackscattering, respectively.

SEMs detect topography because of the different number of secondaryelectrons that escape when the beam is in the space between featurescompared to when the beam is on top of the feature. FIG. 10 shows thatsecondary electrons have trouble escaping from a space (especially if itis small), making spaces appear relatively dark. When an electron beamis focused to a spot in a space between lines, scattered electronsinteract with feature 815 which absorbs some of the escaping secondaryelectrons. The detected secondary electron signal is reduced as the beamapproaches the feature edge within the space.

The absorption by the step (i.e. feature 815) can be modeled to producea prediction of the shape of the linescan in the space region. If alarge feature has a left edge 815-1 at x=0, with the feature 815 to theright (positive x), the detected secondary electron signal as a functionof position (SE(x)) will be given by EQUATION 6 below:

$\begin{matrix}{{{{For}\mspace{14mu} x} < 0},{\frac{{SE}(x)}{{SE}\left( {- \infty} \right)} = {1 + {\alpha_{f}e^{x/\sigma_{j}}} - {\alpha_{b}e^{x/\sigma_{b}}}}}} & {{EQUATION}\mspace{14mu} 6}\end{matrix}$

where α_(f) is the fraction of forward scatter secondary electronsabsorbed by the step and α_(b) is the fraction of backscatter secondaryelectrons absorbed by the step.

However, when the beam is on top of feature 815, the interaction of thescattered electrons with the feature is very different, as accounted forin EQUATION 7 below. As illustrated in FIG. 8, two phenomena occur aswhen the beam is closer to the edge compared to further away. First,secondary electrons from both forward and backscattered electrons canmore easily escape out of the edge 815-1. This causes the edge bloomalready discussed above. To account for this effect, a positive termα_(e)e^(−x/σ) ^(e) is added to account for the enhanced escape offorward-scattered secondary electrons where σ_(e) is very similar to theforward scatter range of the step material. Additionally, theinteraction volume itself decreases when the beam is near the edge815-1, so that there are fewer secondary electrons being generated.Thus, the term α_(v)e^(−x/σ) ^(v) where σ_(v)<σ_(e) is subtracted togive EQUATION 7 below which is the linescan expression for the top ofthe large feature 815:

$\begin{matrix}{{{{For}\mspace{14mu} x} > 0},{\frac{{SE}(x)}{{SE}(\infty)} = {1 + {\alpha_{e}e^{{- x}/\sigma_{e}}} - {\alpha_{v}e^{{- x}/\sigma_{v}}}}}} & {{EQUATION}\mspace{14mu} 7}\end{matrix}$

FIG. 11 shows an example of the result for this model. Morespecifically, FIG. 11 shows a predicted linescan of a left-facing resiststep 815 (large feature with left edge 815-1 at x=0) on a substrate suchas a silicon wafer. The calibrated model 1105 is superimposed on therigorous Monte Carlo simulation results 1110. The calibrated model 1105agrees so closely with the Monte Carlo simulation results 1110 that thetwo curves appear together almost as one line.

The above discussion involves modelling an isolated left-facing edge815-1. Adapting the model to include a right-facing edge involvestranslating and reversing the edge and adding the resulting secondaries(i.e., secondary electrons). Some complications arise if the two edgesare close enough to interact, resulting in additional terms.Additionally, the impact of non-vertical sidewalls and rounded cornersat the top and bottom of the feature edge may be included in the model(FIG. 12).

FIG. 12 shows a representative predicted linescan of a pattern of resistlines and spaces on a silicon wafer. The calibrated model 1205 issuperimposed on the rigorous Monte Carlo simulation results 1210. Again,the calibrated model 1205 agrees so closely with the Monte Carlosimulation results 1110 that the two curves appear together almost asone line. A final model (ALM) includes 15 parameters that depend on theproperties of the materials of the wafer and feature, and the beamvoltage. To validate the model and to calibrate these parameters,rigorous first principle Monte Carlo simulations can be used to generatelinescans for different materials and feature geometries. The ALM canthen be fit to the Monte Carlo results, producing best-fit values of the15 unknown parameters.

6. Inverse Linescan Model

Linescan or image models, such as the analytical linescan model (ALM)discussed above, predict an image or the shape of an image linescan fora particular pattern structure (such as a feature on a wafer). The ALMsolves a forward modelling problem wherein the model receives geometryinformation for the particular feature as input, and provides thepredicted shape of a respective SEM linescan of the particular featureas output.

In contrast to ALM, the disclosed edge detection system 700 includes areverse model that receives as input “measured linescan information”from SEM 701 that describes a particular feature on the wafer. Inresponse to the measured linescan information describing the particularfeature, edge detection system 700 employs its reverse model to generateas output “feature geometry information” that describes the featuregeometry that would produce the measured linescan. Advantageously, edgedetection system 700 has been found to be effective even when themeasured linescan information from SEM 701 includes a significant amountof image noise. In one embodiment, the outputted feature geometryinformation includes at least feature width. In another embodiment, theoutputted feature information includes feature width and/or othergeometry descriptors relative to the geometry of the particular feature,such as sidewall angle, feature thickness, top corner rounding, orbottom footing. It is noted that a feature disposed on a semiconductorwafer is an example of one particular type of pattern structure to whichthe disclosed technology applies.

Like many models of imaging systems, the ALM is inherently nonlinear. Toaddress the nonlinear nature of the ALM, edge detection system 700numerically inverts the ALM or a similar forward model and fits theresulting inverse linescan model to a measured linescan to detectfeature edges (e.g. to estimate the feature geometry on the wafer). Thedisclosed edge detection system apparatus and edge detection processinclude the ability to detect and measure feature roughness. Thedisclosed apparatus and methodology may apply as well to otherapplications in general CD metrology of 1D or 2D features, such as theprecise measurement of feature width (CD) and edge position orplacement.

It is first noted that the ALM (and similar models as well) has twotypes of input parameters, namely material-dependent parameters andgeometry parameters. Material-dependent parameters include parameterssuch as forward and backscatter distances, while geometry parametersinclude parameters such as feature width and pitch. In one embodiment,for a repeated edge detection application, the material parameters willbe fixed and only the geometry parameters will vary. In the simplestcase (that is, for simple edge detection), it is assumed that only theedge positions for the feature are changing, such that sidewall angle,corner rounding, etc., are assumed to be constant. Thus, the use of alinescan model for edge detection in edge detection system 700 involvestwo steps: 1) calibrating the parameters that are assumed to be constantacross the entire image, and then 2) finding the feature edge positionsthat provide a best fit of the measured linescan to the linescan modelfor each measurement.

In one embodiment, in the first step, calibration is accomplished bycomparing the linescan model to rigorous Monte Carlo simulations. Thegoal in this step is to find material parameters over the needed rangeof applications, and to ensure the fitting is adequate for the neededrange of feature geometries. When finished, this calibrated linescanmodel can serve as the starting point for the generation of an inverselinescan model. The Inverse Linescan Model (ILM) should be calibrated tothe specific SEM images that are to be measured. Since image grayscalevalues are only proportional to secondary electron signals, at the veryleast a mapping to grayscale values is required. In real-worldapplications, material properties in the experimental measurement willnot be identical to those assumed in the Monte Carlo simulations suchthat some calibration of those parameters will also be required.

7. Calibration of the Inverse Linescan Model

Before using the ILM for edge detection, the ILM is first calibrated.Some parameters of the model (such as material-dependent parameters) areassumed to be constant for the entire image. However, geometryparameters, such as the positions of the edges, feature width and pitch,are assumed to vary for every linescan. The goal of ILM calibration isto determine the parameters that are constant for the whole image,regardless of the exact positions of the feature edges. It is a furthergoal of ILM calibration to accurately determine these parameters in thepresence of image noise. These goals are accomplished by averaging alongan axis of symmetry for the feature being measured, thus averaging outboth the image noise and the actual feature roughness.

By averaging the linescan along an axis of symmetry (such as thedirection parallel to a long line or space feature), information aboutthe actual edge positions is lost, but information about the materialparameters of the linescan model remain. Further, noise in the image ismostly averaged out in this way. Calibrating the ILM to the averagelinescan produces a set of material parameters (or any parametersassumed constant throughout the image) specific to this image.

Many features to be measured exhibit an axis of symmetry appropriate forILM calibration. For example, a vertical edge has a vertical axis ofsymmetry. Averaging all pixels in a vertical column of pixels from theimage will average away all vertical variation, leaving only horizontalinformation, in a direction perpendicular to the edge of the feature.The result of this averaging is a one-dimensional linescan called theaverage linescan. Likewise, a nominally circular contact hole or pillaris ideally radially symmetric. Averaging through polar angle about thecenter of the feature will produce an average linescan that removesnoise and roughness from the image. An elliptical hole shape can also beso averaged by compressing or expanding the pixel size in one directionin proportion to the ratio of major to minor axes of the ellipse. Otheraxes of symmetry exist for other features as well.

One measured image (for example, one SEM image) may contain one or morefeatures in the image. For example, FIG. 1A shows multiple vertical linefeatures and multiple vertical space features. FIG. 1B shows multiplecontact holes. For such a case, each feature can be separately averagedalong an axis of symmetry to form an average linescan for that feature.For the example of FIG. 1A, the SEM image can be partitioned intovertical stripes, each stripe containing only one line feature, wherethe stripe extends horizontally from approximately the center of onespace to approximately the center of the next space. For the example ofFIG. 1B, the image can be partitioned into separate rectangular regions,each containing exactly one contact hole with the center of the contacthole approximately coinciding with the center of the rectangular region.The averaged linescan for that contact hole is then determined from thatrectangular region of the image. Alternately, each of the averagedlinescans from each feature in an image can themselves be averagedtogether to form a single averaged linescan applicable to the entireimage.

For a repeated edge detection application (such as the detection of allthe edges on a single SEM image), the material parameters will be fixedand only the geometry parameters will vary. In the simplest case (thatis, for simple edge detection), one can assume that only the edgepositions for the feature are changing, so that feature thickness,sidewall angle, corner rounding, etc., are assumed constant. Thus, theuse of the ILM for edge detection will involve two steps: calibratingone time for the parameters that are assumed to be constant (i.e.,material and fixed geometry properties) using the average linescan, andthen finding the feature edge positions that provide a best fit of themeasured linescan to the linescan model for each linescan. Optionally,calibration is first accomplished by comparison of the linescan model torigorous Monte Carlo simulations, as has been previously described. Thegoal of this initial step is to find material parameters over the neededrange of applications, and to ensure the model is adequate for theneeded range of feature geometries. When finished, this partiallycalibrated linescan model must still be fully calibrated to the specificSEM images that are to be measured using the average linescan.

Once the ILM has been calibrated to the given SEM image or sets ofimages, it is then used to detect edges. Due to the non-linear nature oflinescan models such as the ALM model, numerical inversion is needed,for example using non-linear least-square regression to find the valuesof the left and right edge positions that best fit the model to thedata. For simpler linescan models, a linear least-squares fit may bepossible. Other means of “best fit” are also known in the art. The ILMas an edge detector allows the detection of edges in a high noiseenvironment without the use of filters. FIGS. 13A and 13B demonstratethe reliable detection of edges for a very noisy image without the useof any filtering or image smoothing. More particularly, FIG. 13A is anoriginal SEM image of a pattern structure that exhibits 18 nm lines andspaces before edge detection with an ILM. FIG. 13B is the same imageafter edge detection using an ILM.

Gaussian filters are common image smoothing filters designed to reducenoise in an image. Other filters such as box filters and median filtersare also commonly used for this purpose. To illustrate the impact ofimage filtering on roughness measurement, TABLE 1 below shows themeasured 36 linewidth roughness (LWR) as a function of Gaussian filterx- and y-width (in pixels). For each case, the ILM edge detection methodwas used, so that the difference in the resulting LWR is only a functionof the image filter parameters. The range is almost a factor of two,showing that many different roughness measurements can be obtained basedon the arbitrary choice of filter parameters. In all cases, the ILM edgedetection was used. If a conventional threshold edge detection method isused, the range of resulting 3σ roughness values is much greater (TABLE2). Similar results are obtained if other filter types (box or median,for example) are used.

TABLE 1 The raw (biased) 3σ LWR (nm) as a function of Gaussian filter x-and y-width (in pixels), using ILM edge detection. y-width = 1 y-width =2 y-width = 3 y-width = 4 x-width = 1  4.99 4.67 4.03 3.82 x-width = 3 4.92 4.02 3.48 3.28 x-width = 5  4.85 3.82 3.28 3.00 x-width = 7  4.793.69 3.13 2.84 x-width = 9  4.73 3.59 3.08 2.80 x-width = 11 4.68 3.543.07 2.80

TABLE 2 The raw (biased) 3σ LWR (nm) as a function of Gaussian filter x-and y-width (in pixels), using conventional threshold edge detection.y-width = 1 y-width = 2 y-width = 3 y-width = 4 x-width = 1  11.17 8.527.28 x-width = 3  9.58 5.22 4.02 3.72 x-width = 5  8.12 4.62 3.83 3.49x-width = 7  7.44 4.50 3.78 3.42 x-width = 9  7.03 4.45 3.77 3.41x-width = 11 6.77 4.44 3.77 3.41

While the arbitrary choice of image filter parameters has a large impacton the measurement of roughness of the pattern structure, the impact ofthreshold value depends on the specific edge detection method used. Forthe case of a simple threshold edge detection after image filtering,there is one threshold value that minimizes the 3σ roughness measured,with other values changing the roughness quite dramatically (see FIG.14). For the case of the ILM, the choice of threshold has almost noimpact on the measured LWR (in FIG. 14, the LWR varies from 5.00 nm to4.95 nm as the threshold is changed from 0.25 to 0.75). Thus, for theconventional prior art method of detecting edges the arbitrary choice ofthreshold value can cause a large variation in the measured roughness.For the ILM, there are essentially no arbitrary choices that affect themeasurement of roughness.

While the disclosed ILM system achieves accurate detection of edges inthe presence of high levels of noise, the noise still adds to themeasured roughness. For a linescan of a given edge slope, uncertainty inthe grayscale values near the line edge translates directly intouncertainty in the edge position. A major difference, though, is thatthe impact of noise can be measured for the case without filtering. Thenoise floor of an unfiltered image can be subtracted out from the PSD(power spectral density), producing an unbiased estimate of the PSD (andthus the roughness). For the case of a filtered image, the noise flooris mostly smeared away, so that it cannot be detected, measured, orremoved.

FIGS. 15A and 15B show LER power spectral densities from many roughfeatures with right and left edges combined separately. Morespecifically, FIG. 15A shows raw PSDs after edge detection using thedisclosed ILM technology, while FIG. 15B shows PSDs after noisesubtraction.

Consider the results shown in FIG. 15A, where the line-edge roughness(LER) for the left and right edges of a feature on a pattern structureare compared. The raw PSDs indicate that the two edges behavedifferently. However, these differences are an artifact of the SEM,caused by a scan-direction asymmetry (such as charging) that makes theright linescan slope lower than the left linescan slope. In fact, thereis no difference between right and left edge on the wafer for thissample. By measuring the noise floor for each edge separately,subtracting the noise produces a common left/right LER (FIG. 15B) thatis an unbiased estimate of the true PSD.

Once the noise has been subtracted, reliable analysis of the PSD canlead to reliable estimates of the important roughness parameters, suchas the zero-frequency PSD(0), the correlation length ξ, and theroughness exponent H. The unbiased 3σ roughness can also be obtained.Without removing the noise, extraction of these parameters from theempirical PSD is problematic and prone to systematic errors.

8. Unbiased Measurement of PSD

The biggest impediment to accurate roughness measurement is noise in theCD-SEM image. Among other noise sources, SEM images suffer from shotnoise, where the number of electrons detected for a given pixel variesrandomly. For the expected Poisson distribution, the variance in thenumber of electrons detected for a given pixel of the image is equal tothe expected number of electrons detected for that pixel. Since thenumber of detected electrons is proportional to the number of electronsthat impinge on the sample location represented by that pixel, relativeamount of noise can be reduced by increasing the electron dose that thesample is subjected to. For some types of samples, electron dose can beincreased with few consequences. But for other types of samples (such asphotoresist), high electron dose leads to sample damage (resist lineslimming, for example). Other types of samples, such as biologicalspecimens, can also suffer from electron damage. Thus, to prevent sampledamage electron dose is kept as low as possible, where the lowest dosepossible is limited by the noise in the resulting image.

FIG. 16 shows portions of three SEM images of nominally the samelithographic features taken at different electron doses. Morespecifically, FIG. 16 shows portions of SEM images of nominallyidentical resist features with 2, 8, and 32 frames of integration(respectively, from left to right). Doubling the frames of integrationdoubles the electron dose per pixel. Since the dose is increased by afactor of 4 in each case, the noise goes down by a factor of 2.

SEM image noise adds to the actual roughness of the patterns on thewafer to produce a measured roughness that is biased higher. Typically,we obtain a biased roughness as given by EQUATION 8A.σ_(biased) ²=σ_(unbiased) ²+σ_(noise) ²  EQUATION 8A

where σ_(biased) is the roughness measured directly from the SEM image,σ_(unbiased) is the unbiased roughness (that is, the true roughness ofthe wafer features), and σ_(noise) is the random error in detected edgeposition (or linewidth) due to noise in the SEM imaging and edgedetection. EQUATION 8A assumes that the noise is statisticallyindependent of the roughness on the feature being measured. If this isnot the case, more complicated noise models can be used, as furtherdescribed below. Since an unbiased estimate of the feature roughness isdesired, the measured roughness can be corrected by subtracting anestimate of the noise term.

Pixel noise in the SEM creates edge detection noise depending on theshape of the expected linescan for the feature. For example, FIG. 17Ashows a typical linescan (grayscale value versus horizontal position,g(x)) for a line feature on a wafer when there is an extremely largenumber of electrons so that the pixel noise is negligible. The result isthe “expected” linescan, that is, the expectation value of the linescansignal from a statistical perspective. By defining a threshold grayscalelevel, the edge position can be determined. But noise in the grayscalevalues results in noise in the detected edge position. For a givengrayscale noise σ_(gray), the edge position uncertainty σ_(noise) willdepend on the slope of the linescan at the edge dg/dx. For small levelsof noise,

$\begin{matrix}{\sigma_{noise} \sim \frac{\sigma_{gray}}{d{g/d}x}} & \left( {8B} \right)\end{matrix}$Thus, the level of edge detection noise is a function of the pixelgrayscale noise and the slope of the linescan at the feature edge.

This equation 8B is strictly only valid for small levels of noise and aninfinitely small pixel size. To explore the impact of greater amounts ofnoise and a non-zero pixel size, simulation of SEM images was employed.Perfectly smooth lines and spaces (25 nm width, 50 nm pitch) were usedas inputs to the Analytical Linescan Model in order to create syntheticSEM images. Then the resulting grayscale values (which range from 0 to255) of each pixel were treated as the mean of a normal distributionwith a given standard deviation (σ_(gray)) and a random grayscale numberwas assigned to each pixel drawn from this normal distribution. TheseSEM images were then treated as experimental SEM images and measuredusing an inverse linescan model to detect the edge positions of eachfeature. The 1-sigma LER measured from these images is the detected edgeposition uncertainty due to the grayscale pixel noise. FIG. 17B showsthe 1-sigma uncertainty in edge detection position for these perfectlysmooth features in the presence of grayscale noise. In this graph, theedge detection noise, for three different X pixel sizes, is plotted as afunction of grayscale noise for simulated synthetic SEM images (averageof 100 images, each with 20 dense lines/space features of width 25 nmand pitch 50 nm). The edge detection used an inverse linescan model andthe resulting line-edge roughness of the features was considered to bethe edge detection noise. The result is somewhat nonlinear, with higherlevels of pixel noise producing ever greater edge detection noise.Further, smaller X pixel sizes produce lower levels of edge detectionnoise. In fact, the edge detection variance σ_(noise) ² is directlyproportional to the X pixel size for low levels of grayscale noise.

Pixel noise is not the only source of edge detection noise. Duringoperation the electron beam is scanned from left to right using beamsteering electronics. Errors in the beam steering can place the beam atan incorrect position, which produces an edge error. Charging of thesample during electron exposure will deflect the beam to an incorrectposition. While some of the charging effects will be systematic, therewill also be random or pseudo-random components that will appear asrandom variation in the detected edge position.

While several approaches for estimating the SEM edge position noise andsubtracting it out have been proposed in the prior art, these approacheshave not proven successful for today's small feature sizes and highlevels of SEM image noise. The problem is the lack of edge detectionrobustness in the presence of high image noise. More particularly, whennoise levels are high, edge detection algorithms often fail to find theedge. The solution to this problem is typically to filter the image,smoothing out the high frequency noise. For example, if a Gaussian 7×3filter is applied to the image, then for each rectangular region of theimage 7 pixels wide and 3 pixels tall, the grayscale values for eachpixel are multiplied by a Gaussian weight and then averaged together.The result is assigned to the center pixel of the rectangle. Box (mean)filters and median filters can also be used and produce similar results.This smoothing makes edge detection significantly more robust when imagenoise is high. FIG. 17C shows an example of using a simple thresholdedge detection algorithm with image filtering in the right image andwithout image filtering in the left image. Without image filtering, theedge detection algorithm is mostly detecting the noise in the image anddoes not reliably find the edge.

The use of image filtering can have a large effect on the resulting PSDand measured roughness. FIG. 18 shows the impact of two different imagefilters on the PSD obtained from a collection of 30 images, eachcontaining 12 features. All images were measured using an inverselinescan model for edge detection. The power spectral densities wereaveraged from these 360 rough features with images preprocessed using a7×2 or 7×3 Gaussian filter, or not filtered at all, as labelled in thedrawing. As can be appreciated, the high-frequency region is greatlyaffected by filtering. But even the low frequency region of the PSDshows a noticeable change when using a smoothing filter. Filtering inthe y-direction smoothes out high-frequency roughness. Filtering in thex-direction lowers the slope of the linescan, which can affect measuredlow-frequency roughness. As will be described next, the use of imagefiltering makes measurement and subtraction of image noise impossible.

If edge detection without image filtering can be accomplished, noisemeasurement and subtraction can be achieved by contrasting the PSDbehavior of the noise with the PSD behavior of the actual waferfeatures. We expect resist features (as well as after-etch features) tohave a PSD behavior as shown in FIG. 19 as the “True PSD” (and alsoshown earlier in FIG. 4). Correlations along the length of the featureedge reduce high-frequency roughness so that the roughness becomes verysmall over very short length scales. SEM image noise, on the other hand,can often be assumed to be white noise, so that the noise PSD is flatover all frequencies. Other models of the SEM image noise are alsopossible, for example using linescan-to-linescan correlation to describethe noise, as further described below. Thus, at a high enough frequencythe measured PSD will be dominated by image noise and not actual featureroughness (the so-called “noise floor”). Given the grid size along thelength of the line (Ay), SEM edge detection white noise affects the PSDaccording to EQUATION 9 below:PSD _(biased)(f)=PSD _(unbiased)(f)+σ_(noise) ² Δy  EQUATION 9

Thus, measurement of the high-frequency PSD (in the absence of any imagefiltering) provides a measurement of the SEM edge detection noise. FIG.19 illustrates this approach for the case of a white SEM noise model.Clearly, this approach to noise subtraction cannot be used on PSDscoming from images that have been filtered, because such filteringremoves the high-frequency noise floor (see FIG. 18).

EQUATION 9 assumes a white noise model, where the noise found in anypixel of the image is independent of the noise found in any other pixel.This may not always be the case. For example, the noise in each pixelmay be correlated somewhat with its nearest neighbors, affectingσ_(gray) in equation 8B. Alternately, the grayscale slope in equation 8Bmay be correlated from one row of pixels to the next, possibly caused bythe interaction volume of the electrons as shown in FIG. 8. If acorrelation model is assumed or measured, a suitable noise expressionfor the PSD can be used to replace EQUATION 9, as further describedbelow.

FIG. 19 shows one embodiment of the noise subtraction process of thedisclosed edge detection apparatus and method. In the disclosed edgedetection method, the method first detects the positions of the edgesusing the ILM without the use of any image filtering (for example, usingan inverse linescan method). From these detected edges a biased PSD isobtained, which is the sum of the actual wafer roughness PSD and the SEMnoise PSD. Using a model for the SEM image noise (such as a constantwhite noise PSD), the amount of noise is determined by measuring thenoise floor in the high-frequency portion of the measured PSD. The true(unbiased) PSD is obtained by subtracting the noise level from theas-measured (biased) PSD. The key to using the above approach of noisesubtraction for obtaining an unbiased PSD (and thus unbiased estimatesof the parameters σ_(LWR)(∞), PSD(0), and is to robustly detect edgeswithout the use of image filtering. This can be accomplished using aninverse linescan model. An inverse linescan model was used to generatethe no-filter PSD data shown in FIG. 18.

An example method for subtracting white noise will now be described.First, edges are detected from a SEM image without using any imagefiltering (for example, using an inverse linescan model). The powerspectral densities of one or more edges are calculated in the usual way.Since the PSD of a single edge is quite noisy, it is extremely valuableto measure many edges and average the PSDs. Often hundreds or thousandsof edges are measured and their PSDs averaged. This averaged PSD iscalled the biased PSD. From the average biased PSD, the highestfrequencies are inspected to determine if a flat noise floor isobserved. Such a noise floor is observed whenever the y pixel size issufficiently smaller than the correlation length of the true roughness.Typically, a y-pixel size that is 20% of the correlation length orsmaller is adequate. If a noise floor is observed, the average PSD valuein the flat region is calculated. This is the noise floor. This numberis then subtracted from the biased PSD at every frequency to produce theunbiased PSD. The biased PSD is our best estimate of the true PSD of theroughness on the wafer.

Other SEM errors can influence the measurement of roughness PSD as well.For example, SEM field distortion can artificially increase thelow-frequency PSD for LER and PPR, though it has little impact on LWR.Background intensity variation in the SEM can also cause an increase inthe measured low-frequency PSD, including LWR as well as LER and PPR. Ifthese variations can be measured, they can potentially be subtractedout, producing the best possible unbiased estimate of the PSD and itsparameters. By averaging the results of many SEM images where the onlycommon aspect of the measurements is the SEM used, determination of SEMimage distortion and background intensity variation can be made.

Further, the SEM noise itself can vary across the SEM image field. Thus,unbiasing of the roughness measurement can also include the detection ofnoise that varies across the SEM image field and unbiasing differentpoints in the SEM image field according to its measured noise bias atthose points in the SEM image field.

9. Sensitivity to Metrology Tool Settings

Not all noise in measured PSDs is white noise. White noise occurs whenthe measurement noise of the edge position from each linescan iscompletely independent of all other linescans (and in particular, itsnearest neighbors). White noise occurs in the absence of correlationsthat connect the errors in one linescan to the errors in the neighboringlinescans. Any small correlations in edge errors along the length of theline would cause “pink noise”, a noise signature that is not perfectlyflat over the entire frequency region.

The settings of the SEM metrology tool can impact the measured roughnessof a feature in a pattern structure. These settings include themagnification and pixel size of SEM 701. These two parameters can bechanged independently by changing the number of pixels in the image(from 512×512 to 2048×2048, for example). Additionally, the number offrames of integration (the electron dose) when capturing an SEM imagecan be adjusted. To study the impact of this setting, the number offrames of integration can be varied from 2 to 32, representing a 16×variation in electron dose, for example.

Total electron dose is directly proportional to the number of frames ofintegration. Thus, shot noise and its impact on edge detection noise isexpected to be proportional to the square root of the number of framesof integration. FIG. 20 shows PSDs of a particular resist feature typeon a given wafer, measured with different numbers of frames ofintegration. In this case, the PSDs correspond to 18 nm resist lines andspaces where only the number of frames of integration was varied. SEMconditions used were 500 eV, 49 images per condition, 21 features perimage, pixel size=0.8 nm square, and image size=1024×1024 pixels. Thecases of 8 or more frames of integration produce PSDs that exhibit afairly flat high-frequency noise region. For 2 and 4 frames ofintegration the noise region is noticeably sloped. Thus, the assumptionof white SEM noise is only approximately true, and becomes a moreaccurate assumption as the number of frames of integration increases andnoise level decreases. This observation has been borne out in othercircumstances: High noise cases are more likely to exhibit non-flatnoise floors.

FIG. 21 shows the biased and unbiased values of the 3σ linewidthroughness measured as a function of the number of frames of integration.All conditions were the same as described in FIG. 20, and error barsrepresent 95% confidence interval estimates. The biased roughness variesfrom 8.83 nm at two frames of integration to 5.68 nm at 8 frames and3.98 nm at 32 frames. The unbiased roughness, on the other hand, isfairly stable after 6 frames of integration, varying from 5.25 nm at twoframes of integration to 3.25 nm at 8 frames and 3.11 nm at 32 frames.While the biased roughness is 43% higher at 8 frames compared to 32, theunbiased roughness is only 4% higher at 8 frames compared to 32. Sincethe assumption of white SEM noise is not very accurate at 2 and 4 framesof integration, the noise subtraction of the unbiased measurement usinga white noise model is not completely successful at these very lowframes of integration. A correlated noise model can produce better noisesubtraction especially for the low frames of integration, as is morefully described below. While the results shown are for LWR, similarresults are obtained for the measurement of line edge roughness (LER)and pattern placement roughness (PPR).

One possible cause of correlations in edge noise would be correlationsin the pixel noise. To test this possibility, isolated edges weremeasured in the CD-SEM. The edge allows the SEM to perform its imagingfunctions in a typical way, but at a distance left or right from theedge the field is flat and featureless. In this region the onlyvariation in pixel grayscale values comes from image noise. Thecorrelation coefficient between neighboring pixels can then becalculated. Performing these calculations, the average correlationbetween neighboring pixels in the x-direction was 0.12, but the averagecorrelation in the y-direction was only 0.01, essentially zero. Thesecorrelations coefficients were determined for edges measured at 2 to 32frames of integration. There was little variation in the pixel-to-pixelcorrelation as a function of the number of frames of integration. Thus,correlated pixel noise is not responsible for the pink noise observed atlow frames of integration. However, it is possible that the linescanslope in equation 8B is responsible for the noise correlations.

A possible cause of noise correlations in the linescan slope comes fromthe interaction of the beam with the sample. Electrons striking thesample undergo a number of processes that depend on the energy of theelectron and the material properties of the sample. Electrons scatteroff the atoms of the sample material, release energy, change direction,and often generate a cascade of secondary electrons by ionizing thesample atoms. Occasionally electrons ricochet backwards off the atomnucleus and exit out of the sample (called backscatter electrons). Someof the lower energy secondary electrons can also escape out of thesample (frequently through the edges of a feature, see FIGS. 8A and 8B).The way in which a SEM forms an image is by detecting the number ofsecondary electrons and/or backscatter electrons that escape the samplefor each beam position.

When forming an image using an SEM, a small spot of electrons dwells ata specific point on the sample (i.e., a pixel) while the number ofescaping secondary electrons is counted by the secondary electrondetector. When the spot is a long way from a feature edge, as in FIG.8A, the number of detected secondary electrons 805 is small (and thepixel is dark). When the spot is near a feature edge, as in FIG. 8B,secondary electrons 805 from the interaction volume readily escape fromthe feature edge producing a bright pixel.

The interaction volume of the electrons can be one to a few tens ofnanometers in diameter, depending on the beam voltage and the samplematerial properties. This interaction volume means that electronsimpinging on one spot on the sample are influenced by the sample shapeover a range determined by the interaction volume. Thus, the slope ofthe linescan at one row of pixels will not be independent of the slopeof the linescan at neighboring pixels whenever the interaction volumeradius is greater than the y pixel size. This dependency can be thecause of correlations in the noise, with a noise correlation lengthaffected by the electron beam interaction volume.

10. Detecting and Removing Spikes from a Power Spectral Density

In addition to noise interfering with the signal in typical images ofrough features, other errors can be present in the images that have avery different frequency behavior as compared to white noise or pinknoise, and as compared to the roughness being measured. Some such errorsproduce large but narrow spikes in a PSD. FIG. 25A shows one example ofhigh frequency “spikes” that intermittently are found in datasets. Onecause for such spikes can be electrical interference in the scanningelectronics of the imaging tool. If the interference is at a frequencyin a range that allows one or more interfering events within a full scanof the image, this interference can result in a slight but regular“jitter” of the scanning beam position. For highly precise scanning,even a sub-nanometer jitter can result in one or more large spikes inthe measured PSD. Depending on the mechanism, such interference spikesmay be present in the line-edge roughness (LER) and pattern placementroughness (PPR) but not the linewidth roughness (LWR) PSD. Alternately,the interference may cause spikes at the same frequencies in all threePSDs.

For example, electrical interference at a frequency of 50 Hz or 60 Hzcan cause noticeable spikes in a measured PSD when the measurement toolcaptures images at a standard “TV” scan rate or small multiples of thisrate. Additionally, electrical interference at normal audio frequenciescan cause spikes that are visible at higher PSD frequencies in typicalmeasurement tool images.

The presence of spikes in the PSD can be undesirable for a number ofreasons depending on their quantity, their amplitude, and theirfrequency. For the case of high-frequency spikes as seen in FIG. 25A,the spikes can affect the noise removal process described above,resulting in an overestimation of the amount of white or pink noise inthe image.

PSD spikes can be caused by phenomenon other than electricalinterference within the imaging tool. The object being measured mayinclude periodic or semi-periodic structures other than the roughfeatures that are to be measured. For example, a set of verticallyoriented rough features of the object may be on top of a periodic set ofhorizontal features resulting in topography below the rough featuresthat are slightly visible in the image. Such underlying topography canresult in a mid-frequency spike to the PSD (with higher-frequencyharmonics possible as well). FIG. 26 shows an example of thisphenomenon.

Another phenomenon that can give rise to spikes in the PSD would be thepresence of grains of a small size range within the material of thefeatures on the object to be measured. Grains of similar size packedtightly together can produce a nearly periodic appearance that resultsin a spike in the measured PSD.

Roughness measurements can also be performed on images taken ofphotomask features, where said photomasks are used in a lithographyprocess. Photomasks are typically fabricated using a direct-writelithography tool with limitations such as a non-zero address grid andrectangular shots to make up the image. For some features, such as aline oriented at 45 degrees to the direction of the writing grid of thetool used to print the photomask, the result will be small, regularlyspaced jogs along the edge of the photomask feature. These jogs willproduce a spike (or a main spike plus harmonic spikes) in the PSD of themeasured photomask roughness.

Spikes such as those found in FIG. 26 can be very disadvantageous to themeasurement of roughness parameters from the biased or unbiased PSD.FIG. 27A shows how a PSD with spikes can alter the model that is fit tothe unbiased PSD, including modeling parameters such as PSD(0),correlation length, and roughness exponent. In contrast, FIG. 27B showshow a PSD with spikes removed can affect the model that is fit to theunbiased PSD, including modeling parameters such as PSD (0), correlationlength, and roughness exponent.

For these and other reasons, it is desirable to remove spikes in the PSDwhen the cause of those spikes is thought to be from a mechanismdifferent from the mechanisms that give rise to the roughness of thefeatures being measured. In other words, it is desirable to separate thePSD artifacts caused by one mechanism (such as spikes caused byelectrical interference) from the PSD artifacts caused by othermechanisms (such as the stochastic effects that give rise to roughness).This can be done much like the noise removal described above, byrecognizing the different frequency signatures of the differentmechanisms.

As mentioned above, white noise (or pink noise) can be separated fromthe true (unbiased) roughness PSD since the noise frequency signature(flat or near flat at high frequencies) is very different from thefrequency signature of the true roughness (a power-law decreasing athigh frequencies). Likewise these so-called spikes in the PSD havefrequency signatures that are very different from the frequencysignature of the feature roughness itself. In particular, a so-calledspike has a high amplitude over a very narrow frequency range.

A procedure for detecting and removing spikes will now be described.First, the definition of a “spike” can be established as being afrequency response that rises and falls over a frequency range smallerthan a threshold (the “threshold range”) and has a height great than athreshold (the “threshold height”).

Next, a baseline can be established as being the best estimate of thePSD without the spike. For example, the threshold range for spikedetection can be set to three frequency increments in the PSD data(which typically is sampled at a constant frequency increment). Otherthreshold ranges are also possible. A baseline can be determined bysmoothly connecting PSD values separated by the threshold range plus oneincrement (using a straight line on a linear or logarithmic scale, forexample, or by using a model for the expected PSD behavior). Thisbaseline is then subtracted from the actual PSD data within thisthreshold range to arrive at an estimate of the non-baseline PSDbehavior within this frequency range. If the non-baseline PSD behaviorrises to a value greater than the threshold height (expressed either inabsolute terms or as a multiple of the baseline PSD value), then a spikehas been identified. To remove the spike, the calculated baselinebehavior can be used to replace the actual PSD values within thethreshold range. A search for spikes can cover the entire PSD frequencyrange if desired.

The threshold range can be chosen in such a way as to only detect (andpossibly remove) spikes that occur due to specific types of mechanisms.For example, interference at exactly a single frequency will most likelycause a spike in the PSD that is up to two frequency increments wide(since the spike is unlikely to be at a frequency that exactly coincideswith the sampled frequencies of the PSD). A threshold range of two tothree frequency increments wide will be effective in detecting such“single frequency” interference events. A wider threshold range willdetect other, broader-band interference events.

The threshold height can also be adjusted based on the mechanisms thatare desired to be detected. But the minimum threshold height is also afunction of the overall noise in the PSD. Since a PSD measures, bydefinition, the randomness in a random rough sample, PSD measurement isinherently noisy. It is well known that the PSD of a single measuredfeature has a statistical uncertain of 100% (1-sigma). That is, thestatistical uncertainty in any given PSD value at any given frequency is100% for the measurement of a single feature. For that reason, manyfeatures are typically measured and averaged together so that theuncertainty in the PSD can be reduced by one over the square root of thenumber of features being measured.

But for any given number of features measured and averaged, the PSD willhave a statistical uncertainty that is inherent in the sample size. Thethreshold height for spike detection should be chosen to besignificantly higher than the inherent noise level of the PSD.Otherwise, the detection of spikes would be frequently triggered not byphysical spikes but rather by noise in the PSD data. Alternately, thethreshold height can be chosen to be a multiple of the measured orcalculated PSD noise (for example, 5×).

FIG. 25A shows several PSDs (linewidth roughness (LWR PSD 2502),line-edge roughness (LER PSD 2504), and pattern placement roughness PPRPSD 2506) which exhibit several high-frequency spikes (spike artifacts2507). FIG. 25B shows the same PSDs (e.g., LWR PSD 2502 as LWR PSD 2508,LER PSD 2504 as LER PSD 2510, and PPR PSD 2506 as PPR PSD 2512) with thespikes removed using the procedure outlined in the previous paragraphs.For this removal, the threshold range was set to three frequencyincrements, and the threshold height was set to be three times thebaseline PSD value. Effective removal of the spikes was accomplishedusing these settings.

FIGS. 27A and 27B show another case of spike removal, this time formid-frequency spikes. The left-hand graph FIG. 27A shows the PSDs(biased and unbiased) before spike removal. The presence of the spikeshas a deleterious effect on the modeling the PSD and the extraction ofPSD measured values. The right-hand graph, FIG. 27B shows the same PSDswith the spikes removed using the procedure outlined in the previousparagraph. For this removal, the threshold range was set to threefrequency increments, and the threshold height was set to be three timesthe baseline PSD value. Effective removal of the spikes was accomplishedusing these settings. The resulting PSD modeling and PSD measurementmore accurately reflects the feature roughness PSD behavior excludingthe mechanism that gave rise to the spikes.

An alternate procedure of removing spikes will now be described. Spikescan be removed from a PSD by passing the PSD through a low-pass filter.Using well-known techniques, the PSD can be Fourier transformed,multiplied by a low-pass frequency filter, then inverse Fouriertransformed. The cut-off frequency of the low-pass filter can be set toonly filter away spikes narrower than a set limit. Other approaches tolow-pass filtering known in the field can also be applied.

Other methods for detecting and removing spikes based on the differentfrequency characteristics of a spike compared to the more slowly varyingtrue roughness PSD will be known to those skilled in the art.

Referring to FIG. 7, the Information Handling System 750 can be modifiedto include the detection and/or removal of spikes using one of theexemplary methods described here. Information about each detected spike,such as its center frequency, amplitude, area, and/or width, can berecorded and output to Output Device 770. This information can be usefulfor identifying the root cause of the spike formation and thus canassist in the process of reducing or eliminating such root causemechanism.

11. Detection and Measurement of PSD Bumps

Other phenomenon can give rise to PSD behavior that appears as a “bump”in the PSD that otherwise has the typical shape shown in FIG. 3. Suchbumps generally occur at relative low frequencies. These bumps aredistinguished from spikes by covering a relative wide range offrequencies, as opposed to the narrow frequency confines of a spike.FIGS. 28A and 28B show two examples of this so-called bump behavior inPSD, labeled as Bump Type I and Bump Type II.

Bump Type I (FIG. 28A) is a large rise in the low-frequency PSD behaviorabove what would normally be considered the flat low-frequency regimecharacterized by PSD(0). Several mechanisms can give rise to this bump,such as the presence of photomask roughness that is then transferred tothe wafer during a photolithography step. Uncompensated fielddistortions in the imaging tool used to capture the images beingmeasured can also give rise to this kind of bump. Other mechanisms arepossible as well.

Bump Type II (FIG. 28B) occurs at low-to-mid frequencies such that thePSD behavior at frequencies higher and lower than the bump follows theexpected behavior (as seen, for example, in FIG. 3). When this type ofPSD bump is found in the line-edge roughness PSD but not in thelinewidth roughness PSD, the effect is sometimes called “wiggle” sinceit can be noticeable as a wiggle in the feature of the image. Suchwiggle can be caused, for example, from stress or tension in the filmsused to make the features. Photolithography and subtractive etching ofthe film to form the features can relieve stress and allow the relaxedremaining film to wiggle. Other mechanism for causing wiggle are alsopossible.

Like white noise and spikes, bumps in the PSD are thought to arisethrough mechanisms separate from the stochastic mechanism that gave riseto the rest of the PSD. Thus, it is desirable to separate out theeffects of the bump from the rest of the PSD. It is possible to use aprocedure similar to spike detection and removal for bump detection andremoval. However, this approach becomes problematic when the width ofthe bump is large due to the difficulty in defining a baseline PSDbehavior over a large frequency range. While the larger frequency rangeof the bump means it is possible to distinguish bumps from spikes, italso means that different procedures for detecting and measuring bumpsare likely required.

A separate technique of bump detection, measurement, and removalinvolves the use of a model for the bump. Like white noise and pinknoise, the bump model adds directly to the typical PSD of the featureroughness. Thus, the bump model can be fit to the PSD simultaneouslywith the typical PSD model that does not include bump behavior.

A useful form for a bump model is given in Equation 10 below:PSD _(bump)(f)=Ae ^(−(f-f) ^(c) ⁾ ² ^(/2σ) ^(w) ²   (10)

where A is the amplitude of the bump, f_(c) is the center frequency ofthe bump, and σ_(w) is the width of the bump. For a Type I bump (FIG.28A), the center frequency can be zero. Other models may also be used.Alternate parameterizations of the model can also be used, such as thearea and center frequency of the bump.

The area of the bump above the baseline PSD, as determined for examplefrom the best fit model, is a useful measure of the magnitude of thephenomenon that gave rise to the bump. For example, for the case ofwiggle (a Bump Type II example, FIG. 28B), the area represents thevariance of the wiggle that adds to the variance caused by stochasticroughness. In other words, this approach for bump detection andmeasurement allows the total variance of the feature to be separatedinto a wiggle variance plus a stochastic roughness variance.

Referring to FIG. 7, the Information Handling System 750 can be modifiedto include the detection and/or removal of bumps using one of theexemplary methods described here. Information about each detected bump,such as its center frequency, amplitude, area, and/or width, can berecorded and output to Output Device 770. This information can be usefulfor identifying the root cause of the bump formation and thus can assistin the process of reducing or eliminating such root cause mechanism. Bysubtracting the bump behavior from the total PSD, the remaining PSD canbe characterized (using, for example, parameters such as PSD(0),correlation length, and roughness exponent) so that this remaining PSDreflects more accurately the mechanisms that gave rise to the PSDexclusive of the bump mechanism.

Referring now to FIG. 30, an example method 3000 to detect undesiredspikes in a PSD dataset, and for removing spikes in a PSD dataset isillustrated. The method 3000 starts (block 3002) and generates, using animaging device, a set of one or more images, each image of the setincluding an instance of a feature within a respective patternstructure, each image including measured linescan informationcorresponding to the pattern structure that includes noise (block 3004).Next the method proceeds to detect edges of the features within thepattern structure of each image of the set without filtering the images(block 3006) and generates a power spectral density (PSD) datasetrepresenting feature geometry information corresponding to the edgedetection measurements of the set of images (block 3008). If desired, anunbiased PSD data set can be generated from the biased PSD data set bysubtracting SEM noise. Next, the method defines a threshold range and athreshold height (block 3010) and generates a baseline for a portion ofthe PSD dataset, by smoothly connecting a first PSD value of the portionof the PSD dataset to a second PSD value, wherein the first PSD valueand the second PSD value are separated by the threshold range (block3012), determines that a difference between a third PSD value of theportion of the PSD dataset and the baseline is greater than a thresholdheight (block 3014), and replaces the portion of the PSD dataset withthe baseline for the portion of the PSD dataset (block 3016).Thereafter, the method ends (block 3018).

Referring now to FIG. 31, an example method 3100 to model bumps in a PSDdataset is illustrated. The method 3100 starts (block 3102) andgenerates, using an imaging device, a set of one or more images, eachimage of the set including an instance of a feature within a respectivepattern structure, each image including measured linescan informationcorresponding to the pattern structure that includes noise (block 3104).Next the method proceeds to detect edges of the features within thepattern structure of each image of the set without filtering the images(block 3106) and generate a biased power spectral density (PSD) datasetrepresenting feature geometry information corresponding to the edgedetection measurements of the set of images (block 3108). If desired, anunbiased PSD data set can be generated from the biased PSD data set bysubtracting SEM noise. A first bump is evaluated in the PSD dataset tocreate a bump model (block 3110); and fits a typical PSD model and thebump model to the PSD dataset to create a best fit model (block 3112).Thereafter, the method ends (block 3114).

The flowcharts of FIG. 30 and FIG. 31 include the steps that can beperformed using the system 700 depicted in FIG. 7, including certainsteps that can be carried out by the SEM 701 and certain other stepsthat can be carried out by the information handling system (IHS) 750 andits included processor 755 and storage 760, both as described in detailherein. Instructions can be stored in storage 760 that, when executed bythe processor, cause the processor to perform the methods disclosedherein and described by the flowcharts of FIG. 30 and FIG. 31, inanalogous fashion as other instructions stored in storage 760 thatimplement the inverse linescan model metrology tool 765 describedherein.

12. Influence of Pixel Size and Magnification

With respect to the pixel size and magnification employed by SEM 701,FIGS. 22A and 22B show the biased and unbiased power spectral densities(PSDs), respectively, for a pattern of 16 nm lines and spaces fordifferent magnifications and pixel sizes, assuming a white noise model.For a given number of frames of integration, changing the pixel sizechanges the electron dose per unit wafer area and the noise in the SEMimage. Table 3 shows the measured 36 linewidth roughness (LWR), as wellas the other PSD parameters, for these different pixel size andmagnification conditions. Under this range of conditions, the biased LWRvaried by 0.63 nm (14%), while the unbiased LWR varied by only 0.07 nm(2%). The unbiased LWR is essentially unaffected by these metrology toolsettings. Similar results are obtained for the measurement of LER andPPR.

FIGS. 22A and 22B show power spectral densities as a function of pixelsize and magnification. More particularly, FIG. 22A shows the biased LWRPSD and FIG. 22B shows the unbiased LWR PSD after noise has beenmeasured and subtracted off. The SEM conditions for these results used alanding energy of 500 eV, 3 images per condition, and 16 nm resist linesand spaces.

TABLE 4 below shows the measured PSD parameters for the PSDs shown inFIGS. 22A and 22B.

TABLE 4 Biased and unbiased 3σ LWR (nm) measurements as a function ofpixel size and magnification. Pixel Pixel Pixel Pixel Pixel 0.8 nm 0.8nm 0.5 nm 0.5 nm 0.37 nm 82 kX 164 kX 130 kX 264 kX 180 kX Biased LWR5.10 4.99 4.67 4.61 4.47 (3-sigma, nm) Unbiased LWR 3.66 3.65 3.70 3.673.63 (3-sigma, nm) Unbiased LWR 15.95 16.18 17.2 16.25 16.35 PSD(0)(nm³) LWR Correlation 5.08 5.05 5.31 5.11 5.38 Length (nm)

It has been found that the difference between biased and unbiased LWR isnot constant, but varies with metrology tool settings, feature size, andprocess. Likewise, the ratio between biased and unbiased LWR varies withmetrology tool settings, feature size, and process. TABLE 5 below showsthe difference and ratio of biased to unbiased LWR for a variety ofconditions. For these conditions, the ratio of biased to unbiased LWRvaries from 1.09 to 1.66. The difference between biased and unbiased LWRvaries from 0.32 nm to 2.19 nm in this particular example.

TABLE 5 The relationship between biased and unbiased LWR for a varietyof processes. 3σ LWR: 3σ LWR (nm): Process Biased/Unbiased Biased -Unbiased 193i litho, 84 nm pitch, 500 V, 1.20 0.76 512 rect pixels 193ietch, 84 nm pitch, 800 V, 1.14 0.43 512 rect pixels EUV litho, 32 nmpitch, 500 V, 1.39 1.44 2048 0.8 nm pixels EUV litho, 32 nm pitch, 500V, 1.37 1.34 1024 0.8 nm pixels EUV litho, 32 nm pitch, 500 V, 1.26 0.972048 0.5 nm pixels EUV litho, 32 nm pitch, 500 V, 1.26 0.94 1024 0.5 nmpixels EUV litho, 32 nm pitch, 500 V, 1.23 0.84 1024 0.37 nm pixels EUVlitho, 36 nm pitch, 500 V, 1.52 1.86 1024 0.8 nm pixels EUV litho, 32 nmpitch, 500 V, 1.66 2.19 1024 rect pixels EUV etch, 32 nm pitch, 800 V,1.09 0.32 1024 rect pixels

13. Edge Detection Embodiments

FIG. 23 is a flowchart that depicts a representative overall processflow that the disclosed SEM edge detection system employs to detectedges of a pattern structure. For discussion purposes, the processdescribed in the flowchart of FIG. 23 is applied to sample 2400 of FIG.24A. Sample 2400 is a pattern structure that may also be referred to aspattern structure 2400. The flowchart of FIG. 23 includes the stepscarried out by inverse linescan model metrology tool 765 to determinethe edges of the pattern structure.

Process flow commences at start block 2300 of FIG. 23. As seen in FIG.7, an information handling system (IHS) 750 is coupled to SEM 701 toreceive SEM linescan image information from SEM 701. IHS 750 includes aprocessor 755 and storage 760 coupled thereto. Storage 760 may includevolatile system memory and non-volatile permanent memory such as harddrives, solid state storage devices (SSDs) and the like that permanentlystore applications and other information. Storage 760 stores the inverselinescan model (ILM) metrology tool 765 disclosed herein and describedby the flowchart of FIG. 23. SEM 701 includes a controller (not shown)that IHS 760 instructs to perform image acquisition on pattern structure800 and that provides linescan information from SEM 701 to IHS 750.

As per block 2305, SEM 701 sends an SEM image of pattern structure 800to IHS 750, and in response, IHS 750 loads this SEM image into systemmemory within storage 760. IHS 750 preprocesses the pattern structureimage from the SEM 701, as per block 2310. For example, thispreprocessing of the loaded SEM image may include adjusting grayscalevalues and subtracting out background tilts of intensity levels.Optionally, as per block 2315, IHS 750 may perform filtering of theloaded image, although this is generally not preferred.

In the case of a pattern structure such as the vertical lines and spacesseen in the pattern structure 2400 of FIG. 24A, the inverse linescanmetrology tool 765 averages vertically over the axis of symmetry togenerate an average linescan, as per block 2320. An average linescan maybe a grayscale value as a function of horizontal position wherein all ofthe vertical pixels have been averaged together. This averages out muchof the SEM noise contained in the SEM image and produces a linescan thatis more representative of the physical processes that generate alinescan without noise. FIG. 24B shows a single linescan at one Y-pixelposition. FIG. 24C shows the averaged linescan that is generated byaveraging over all Y-pixels.

While the example shown here is for vertical lines and spaces, anypattern with an axis of symmetry can be so processed to produce anaverage linescan. For example, long lines, long spaces, or long isolatededges can be so processed whenever the length of the line is sufficientto allow adequate averaging. Contact holes or pillars, with circular orelliptical symmetry, can also be averaged in a radial direction toproduce an average linescan.

As per block 2325, tool 765 calibrates the inverse linescan model to theaveraged linescan that was obtained in the manner described above. It isnoted that the linescan model includes two kinds of parameters,namely 1) parameters that depend upon the materials and the propertiesof the SEM, and 2) parameters that depend on the geometry of the featureon the sample. Tool 765 can calibrate all of these parameters. Tool 765finds the best fit of the model to the average linescan of FIG. 24C, asper block 2325. The values of the best fit parameters of the model arethen the calibrated values.

That calibrated model is applied to a single linescan as shown in FIG.24B. The best fit of the model to the single linescan of FIG. 24B isfound, however, in this case tool 765 fixes all of the parameters thatrelate to the materials and SEM imaging tool. In this scenario, tool 765varies only the parameters related to the geometry of the feature of thepattern structure in order to find the best fit of the calibrated modelto a single linescan.

In a simplified scenario, the only parameters varied in block 2330 wouldbe the positions of the edges of the feature. In one embodiment, it isassumed that the vertical dimension of the feature exhibits apredetermined thickness and that only the edge positions of the featureare varying. Next, the calibrated inverse linescan model is fit to everysingle horizontal cut through the 2D image of the feature, as per block2330. We take the top horizontal row of pixels, and then the next row ofpixels that are one pixel down, and then the next horizontal row ofpixels down, and so forth. An example of one such single linescan isshown in FIG. 24B. The resulting best fit edge positions are thedetected edges.

After the edges of the feature are detected in the manner describedabove, tool 765 may detect that the sample was rotated slightly duringimage acquisition, resulting in parallel tilted lines (that is, linesthat are not perfectly vertical). Such tilting or rotation maycontribute to inaccuracy of the detected edges by changing the averagelinescan and thus the calibrated ILM. Image rotation can be detected byfitting all the edges in the image to a set of parallel lines anddetermining their slope compared to vertical. If the slope issufficiently different from the vertical case, the rotation should beremoved. One possible criterion would be to compare the pixel positionof the best fit line at the top of the image to the pixel position ofthe best fit line at the bottom of the image. If these pixel positionsdiffer by some threshold, such as two pixels, then the image rotation isconsidered to be sufficiently large that its removal is required.

If such tilting/rotation is detected, as per block 2335, then the priorcalibration is considered to be a first pass calibration and calibrationis repeated. More particularly, if such tiling/rotation is detected, therotation is subtracted out by shifting some rows of pixels to bring theedges into vertical alignment, as per block 2345, and calculating a newaverage linescan. Calibration of the model is then repeated as per block2350 and 2325. Another fitting is performed as well, as per block 2330.Ultimately, tool 765 outputs geometry feature information (such as edgepositions) describing the geometry of the feature that corresponds tothe linescan image information provided to tool 765.

Like image rotation, the roughness of the features themselvescontributes inaccuracies to the calibration of the ILM. Optionally,after a first pass edge detection, each row of pixels can be shifted tonot only subtract out image rotation, but to subtract out the featureroughness as well. The final result after the shifting of each row ofpixels is a vertical edge where the edge position varies by less thanone pixel from a perfect vertical line. These shifted rows of pixels canthen be averaged vertically to produce a more accurate average linescanfor use in ILM calibration.

In actual practice, information handling system 760 may include aninterface 757 coupled between processor 755 and an output device 770such as a display, printer, or other device so that the user may observethe feature edges determined by metrology tool 765. Interface 757 may bea graphics interface, a printer interface, network interface, or otherhardware interface appropriate for the particular type of output device770.

14. Assessing the Quality of Devices using Unbiased RoughnessMeasurements

The measurement of roughness of various pattern structures can be usedto assess the quality of the devices being fabricated. For example, theyield and/or performance of a device might depend on the magnitude ofthe roughness of one or more patterns that make up that device, as wellas the frequency content of the roughness of those patterns.

The use of roughness measurements to assess device quality can be assimple as defining a “specification” for the roughness: the measuredroughness of a specific target pattern must not exceed a specifiedvalue. The specification is set based on its relationship to deviceyield and/or performance. When devices the “meet” the specification(have roughness that is at or below the specification for the targetpatterns being measured), it is known or assumed that the device willhave acceptable yield and performance.

Alternately, the measurement of roughness can serve as an input into amodel that predicts device yield and/or performance. Such models couldbe run in real time, or results could be precomputed and placed in atable for look-up as needed.

For example, stochastic effects that give rise to edge roughness of apattern structure also give rise the catastrophic detects such as themerging of two edges that should remain separate. For a pattern of linesand spaces, two neighboring lines can bridge the space between themcreating a merger across the space. If those two lines arecurrent-carrying wires, the result is a short circuit. If the two edgesof a single line merge the result is a break in the line. If that lineis current-carrying wire, the result is an open circuit. Defects such asthese are labeled “catastrophic” since the occurrence of just one couldrender the device inoperable. Thus, a yield model for roughness couldtake the output of a roughness measurement of a pattern structure andpredict the probability of catastrophic defect, and thus predict astochastic-limited yield.

The use of biased roughness measurements to assess the quality of adevice is problematic since the biased measurement can overestimate orunderestimate the true roughness of the pattern structures that weremeasured. This in turn could lead to underestimation or overestimationof the device impact of that roughness.

Further, the bias in the roughness measurements in not necessarilyfixed. The bias in the measurements can vary from measurement tomeasurement due to variations in the measurement tool, or due tovariations in the pattern structure that do not affect the trueroughness of the pattern.

For these reasons, it is preferred that the assessment of device qualitybe based on unbiased roughness measurements, where random and/orsystematic errors in the measurement of roughness are removed.

Further, some aspects of device quality are sensitive to the frequencycontent of the roughness. Low frequency roughness behaves like an errorin the mean feature width, edge position, or center-line position of thefeature. Mid-frequency roughness can produce, for example, scattering ofelectrons flowing through a metal wire, increasing its resistance. Foran optical waveguide, the roughness frequency that matter most dependson the wavelength of the light passing through the waveguide. For somedevices, roughness at frequencies higher than a certain cut-off may nothave any affect at all on the performance of the device.

Thus, it is desirable to produce an unbiased estimation of the true PSDbehavior of the roughness as well. Integrating the unbiased PSD over acertain frequency range will provide an estimate of the magnitude of theroughness only over that frequency range, ignoring other frequencies ofthe roughness. This roughness over a set frequency range can then beused as a specification, or as an input to a device quality model.

Note that the measured pattern structure or structures need notduplicate exactly the form of the pattern structure or structures thatmake up the device of interest. The only requirement is that the patternstructures that are measured produce roughness measures that arepredictive of device quality.

15. Assessing the Quality of a Process or Material or Tool usingUnbiased Roughness Measurements

Roughness measurements of a pattern structure or structures can be usedto assess the quality of the processes and/or process materials and/orprocess tools used to fabricate that pattern structure. For example,repeated measurements of roughness can be used to determine temporalvariations in roughness or spatial variations in roughness usingstandard assessment techniques.

The assessment of temporal variations in a process parameter is commonlyaccomplished through either time-series analysis or statistical processcontrol (SPC). Both techniques can identify behavior that deviates fromhistorical trends, thus indicating a process variation that might needattention or an action such as a process adjustment. A drift or anabrupt change in the magnitude of the roughness or in its frequencycomponents could indicate a problem with the process, a problem with amaterial, or a problem with a tool used in the fabrication of themeasure pattern structure.

Spatial analysis of roughness can be used to indicate a systematicspatial signature present in the fabrication process. For example, avariation across the substrate (such as a wafer) might indicate aproblem with the uniformity of the etch process even though othermetrics (such as a measurement of the average dimensions of the patternstructures) do not show a similar spatial signature or uniformityproblem. A variation across the exposure field of a lithography toolmight indicate a problem that is similarly unnoticed by othermeasurements.

Changes in a process material, such as a photoresist used in alithography process, can result in a change in the measured patternroughness, including its frequency behavior as exhibited by the powerspectral density or other equivalent measure.

The use of roughness measurements to assess the quality of a process,process material, or process tool can be significantly degraded when theroughness measurements are biased by noise and/or systematic errors inthe measurement to produce a biased roughness measurement.

Temporal and spatial variations in roughness include variations in thefrequency components of the roughness. For example, a spatial variationin the correlation length (determined, for example, by measurement andanalysis of the PSD) might indicate a variation in a temperaturedependent process such as diffusion, which in turn might indicate atemperature uniformity problem during a baking step of the fabricationprocess.

Biased measurement can be higher or lower than the true value, dependingon the source of bias and whether image filtering was used before edgedetection, for example. Further, the bias in the roughness measurementsin not necessarily fixed. The bias in the measurements can vary frommeasurement to measurement due to variations in the measurement tool, ordue to variations in the pattern structure that do not affect the trueroughness of the pattern.

As a result, the use of unbiased measurements for the assessment of thequality of a process, process material, or process tool is highlydesirable.

Note that the measured pattern structure or structures need notduplicate exactly the form of the pattern structure or structures thatmake up the device or devices being fabricated by the process ofinterest. The only requirement is that the pattern structures that aremeasured produce roughness measures that are predictive of process,process material, or process tool quality.

16. Assessing the Quality of a Metrology Tool and Process using UnbiasedRoughness Measurements

The unbiased measurement of roughness necessarily entails thedetermination of the measurement bias, whether that bias is caused byrandom errors such as image noise and edge detection noise, orsystematic errors such as distortion. As a result, the measurement biasis an output of an unbiased measurement of roughness. The determinationof measurement bias can be used to assess the quality of the tool andprocess used to measure roughness.

For a given measurement tool, measurement process, and pattern structureto be measured, the roughness measurement bias should be a fixedquantity. Further, the edge detection metrology noise as well as thesystematic errors such as measurement distortion should individually beconstant. Thus, changes in these quantities could be an indication of achange in the ability to measure these quantities. By tracking thedetected roughness measurement errors over time it is possible to assesschanges in the measurement and to assess the quality of the measurementprocess and/or the measurement tool.

17. Controlling a Fabrication Process using Unbiased RoughnessMeasurements

The above sections describe how unbiased measurements of roughness of apattern structure can be used to assess the quality of the device thatincorporates the pattern structure. Further, the above sections describehow unbiased measurements of roughness of a pattern structure can beused to assess the quality of the process, process materials, andprocess tools used to make the pattern structure. Further, the abovesections describe how unbiased measurements of roughness of a patternstructure can be used to assess the quality of the of the metrology usedto measure said roughness.

Once the quality of the devices, processes, materials, process tools,and measurement tools have been assessed, that assessment can be used tocontrol the fabrication of the devices. Process control is a well-knownapplication of measurement results including feedback control,feed-forward control, and advanced process control (APC) such asrun-to-run control.

Feedback control uses a measurement result (or many measurement results)to determine a change in the fabrication process (such as a change in asetting, a change in a material, or a change in a tool) that would haveproduced a better result had those changes been implemented prior to thefabrication of the measured pattern structure. These changes are thenimplemented for the fabrication of future pattern structures under theassumption that they will affect the desire correction for the futureresults.

Feedforward control uses a measurement result (or many measurementresults) to determine a change in a subsequent process step that couldcompensate for the errors measured in the current process step.

Advanced process control (PAC) or run-to-run control uses feedforward orfeedback loops very quickly so that changes (either forward or backward)can be implemented with very little delay, reducing the amount ofproduct that is fabricated with the process exhibiting the measurederror.

Unbiased roughness measurements enhance the efficacy of each of thesecontrol approaches. While the uses of biased roughness in feedforward,feedback, and PAC control loops is possible, the results are often lessthan desired (and sometimes worse than no control at all) due to thebiases in the measurements. Further, since the bias in the measurementscan change, it is possible that a feedback loop, for example, couldcause process changes in response to a change in measurement noiserather than a change in the actual pattern structure, thus making theprocess more unstable rather than more stable.

While the above embodiments describe the use of unbiased roughnessmeasurements of a pattern structure or structures to assess and improvethe quality of a fabrication process, the same approaches can be used toreduce the cost of manufacturing while keeping the quality of thefabrication process constant. For example, process quality assessmentmight determine that a process using a shorter etch time producesequivalent unbiased roughness measurement results as a process withlonger etch times. Since the shorter etch time results in higher processthroughput, the cost per device is reduced without a reduction inprocess quality. Biased roughness measurement may not provide this sameresult if the bias in the measurement changes with etch time.

As a second example, the results shown in FIG. 21 show that reducing thenumber of frames of integration in a CD-SEM have a significant impact onbiased roughness measurements for all cases, but has very little impacton unbiased roughness measurement at 8 frames or greater. The throughputof a SEM metrology tool is roughly inversely proportional to the numberof frames of integration used, and so the cost of a measurement isroughly proportional to the number of frames of integration used in themeasurement. By reducing the number of frames of integration, say, from32 to 16, a significant reduction in metrology cost (close to a factorof 2) is achieved without loss of metrology precision or accuracy, butonly if unbiased roughness measurements are used.

While the embodiments described above make reference to the measurementof structures found on semiconductor wafers, as used in the manufactureof semiconductor devices, the invention is not limited to theseapplications. The invention can be usefully employed to measure theroughness of feature edges found on flat panel displays,microelectromechanical systems, microfluidic systems, opticalwaveguides, photonic devices, and other electronic, optical, ormechanical devices. Further, the invention can be used to measure thefeature edge characteristics of naturally occurring structures such ascrystals or minerals, or manmade structures such as nanoparticles orother nanostructures. Further, the invention can be used to measure thefeature edge characteristics of biological samples as well.

While the embodiments described above make reference to measurementsusing a scanning electron microscope, the invention is not limited tothat imaging tool. Other imaging tools, such as optical microscopes,stimulated emission and depletion (STED) microscopes, x-ray microscopes,transmission electron microscopes (TEM), focused ion beam microscopes,and helium ion microscopes, can also be used. Other forms ofmicroscopes, such as scanning probe microscopes (atomic forcemicroscopes (AFM) and scanning near-field optical microscopes (SNOM),for example) can be used as well.

While the embodiments described above make reference to top-down imagesof nominally planar pattern structures to measure edge roughness, theinvention is not limited to such pattern structure geometries.Three-dimensional structures, non-flat structures, curved surfaces, ortilted structures can be measured using this invention. Besides edgeroughness, surface roughness can be measured and analyzed using similartechniques as described in this invention.

While the embodiments described above make reference to the measurementof roughness, the invention can be used to make other measurements aswell. For example, highly accurate determination of pattern structureedges can be used in the measurement of feature width, featureplacement, edge placement, and other similar measures. Contours ofmeasured features can be used for many purposes, such as modeling orcontrolling the performance of the measured device. By collecting andstatistically averaging the measurement of many samples, even greateraccuracy (lower uncertainty) can be obtained.

Consistent with the above disclosure, the examples of systems andmethods enumerated in the following clauses are specificallycontemplated and are intended as a non-limiting set of examples.

Clause 1. A method, comprising:

receiving measured linescan information describing a pattern structureof a feature;

applying the received measured linescan information to an inverselinescan model that relates measured linescan information to featuregeometry information;

identifying, based at least in part on the applying the receivedmeasured linescan model to the inverse linescan model, feature geometryinformation that describes a feature that would produce a linescancorresponding to the received measured linescan information;

determining, at least in part using the inverse linescan model, featureedge positions of the identified feature;

analyzing the feature edge positions to determine errors in themanufacture of the pattern structure; and

controlling a lithography tool based on the analysis of the feature edgepositions.

Clause 2. The method of any preceding clause, wherein the lithographytool is controlled by making an adjustment in at least one of focus andexposure dose during the printing of a pattern structure.

Clause 3. The method of any preceding clause, wherein the lithographytool is controlled to improve the uniformity of feature edge positionsfrom wafer to wafer.

Clause 4. The method of any preceding clause, wherein the lithographytool is controlled to improve the spatial uniformity of the feature edgepositions across a wafer.

Clause 5. The method of any preceding clause, wherein the lithographytool is controlled to improve the spatial uniformity of the featurewidth across a wafer.

Clause 6. The method of any preceding clause, wherein the lithographytool is controlled to reduce the pattern placement error of the patternstructure.

Clause 7. The method of any preceding clause, wherein the lithographytool is controlled to center the values of focus and exposure dosewithin the focus-exposure process window.

Clause 8. The method of any preceding clause, wherein the lithographytool is controlled to increase the size of the focus-exposure processwindow.

Clause 9. The method of any preceding clause, wherein the lithographytool is controlled to increase the throughput of the lithography tool.

Clause 10. The method of any preceding clause, wherein the spatialdistribution or other characteristics of the light source of thelithography tool are adjusted to reduce errors in the feature edgepositions.

Clause 11. The method of any preceding clause, wherein the lithographytool is controlled to improve the matching of feature edge positioncharacteristics to other lithography tools, such as spatial signaturesand pattern placement errors.

Clause 12. An edge detection system, comprising:

an imaging device configured to generate a first image of a patternstructure, wherein the first image of the pattern structure includesimage noise; and

a processor, coupled to the imaging device, the processor configured to:

-   -   generate, based at least in part on the first image of the        pattern structure, measured linescan information describing a        pattern structure of a feature;    -   apply the measured linescan information to an inverse linescan        model that relates measured linescan information to feature        geometry information;    -   identify, based at least in part on the applying the measured        linescan model to the inverse linescan model, feature geometry        information that describes a feature that would produce a        linescan corresponding to the received measured linescan        information;    -   determine, at least in part using the inverse linescan model,        feature edge positions of the identified feature;    -   analyze the feature edge positions to determine errors in the        manufacture of the pattern structure; and    -   control a lithography tool based on the analysis of the feature        edge positions.

Clause 13. The edge detection system of any preceding clause, whereinthe lithography tool is controlled by making an adjustment in at leastone of focus and exposure dose during the printing of a patternstructure.

Clause 14. The edge detection system of any preceding clause, whereinthe lithography tool is controlled to improve the uniformity of featureedge positions from wafer to wafer.

Clause 15. The edge detection system of any preceding clause, whereinthe lithography tool is controlled to improve the spatial uniformity ofthe feature edge positions across a wafer.

Clause 16. The edge detection system of any preceding clause, whereinthe lithography tool is controlled to improve the spatial uniformity ofthe feature width across a wafer.

Clause 17. The edge detection system of any preceding clause, whereinthe lithography tool is controlled to reduce the pattern placement errorof the pattern structure.

Clause 18. The edge detection system of any preceding clause, whereinthe lithography tool is controlled to center the values of focus andexposure dose within the focus-exposure process window.

Clause 19. The edge detection system of any preceding clause, whereinthe lithography tool is controlled to improve the matching of featureedge position characteristics to other lithography tools, such asspatial signatures and pattern placement errors.

Clause 20. A tangible, non-transitory computer-readable medium storinginstructions that, when executed, cause a processing device to:

obtain a first image of a pattern structure, wherein the first image ofthe pattern structure includes image noise;

generate, based at least in part on the first image of the patternstructure, measured linescan information describing a pattern structureof a feature;

apply the measured linescan information to an inverse linescan modelthat relates measured linescan information to feature geometryinformation;

identify, based at least in part on the applying the measured linescanmodel to the inverse linescan model, feature geometry information thatdescribes a feature that would produce a linescan corresponding to thereceived measured linescan information;

determine, at least in part using the inverse linescan model, featureedge positions of the identified feature;

analyze the feature edge positions to determine errors in themanufacture of the pattern structure; and

control a lithography tool based on the analysis of the feature edgepositions.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Note that not all of the activities described above in the generaldescription or the examples are required, that a portion of a specificactivity may not be required, and that one or more further activitiescan be performed in addition to those described. Still further, theorder in which activities are listed are not necessarily the order inwhich they are performed.

It can be advantageous to set forth definitions of certain words andphrases used throughout this patent document. The term “communicate,” aswell as derivatives thereof, encompasses both direct and indirectcommunication. The terms “include” and “comprise,” as well asderivatives thereof, mean inclusion without limitation. The term “or” isinclusive, meaning and/or. The phrase “associated with,” as well asderivatives thereof, can mean to include, be included within,interconnect with, contain, be contained within, connect to or with,couple to or with, be communicable with, cooperate with, interleave,juxtapose, be proximate to, be bound to or with, have, have a propertyof, have a relationship to or with, or the like. The phrase “at leastone of,” when used with a list of items, means that differentcombinations of one or more of the listed items can be used, and onlyone item in the list can be needed. For example, “at least one of: A, B,and C” includes any of the following combinations: A, B, C, A and B, Aand C, B and C, and A and B and C.

The description in the present application should not be read asimplying that any particular element, step, or function is an essentialor critical element that must be included in the claim scope. The scopeof patented subject matter is defined only by the allowed claims.Moreover, none of the claims invokes 35 U.S.C. § 112(f) with respect toany of the appended claims or claim elements unless the exact words“means for” or “step for” are explicitly used in the particular claim,followed by a participle phrase identifying a function. Use of termssuch as (but not limited to) “mechanism,” “module,” “device,” “unit,”“component,” “element,” “member,” “apparatus,” “machine,” “system,”“processor,” or “controller” within a claim is understood and intendedto refer to structures known to those skilled in the relevant art, asfurther modified or enhanced by the features of the claims themselves,and is not intended to invoke 35 U.S.C. § 112(f).

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any feature(s) that cancause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature of any or all the claims.

After reading the specification, skilled artisans will appreciate thatcertain features are, for clarity, described herein in the context ofseparate embodiments, can also be provided in combination in a singleembodiment. Conversely, various features that are, for brevity,described in the context of a single embodiment, can also be providedseparately or in any subcombination. Further, references to valuesstated in ranges include each and every value within that range.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. A method, comprising: receiving measured linescaninformation describing a pattern structure of a feature; applying thereceived measured linescan information to an inverse linescan model thatrelates measured linescan information to feature geometry information;identifying, based at least in part on the applying the receivedmeasured linescan model to the inverse linescan model, feature geometryinformation that describes a feature that would produce a linescancorresponding to the received measured linescan information;determining, at least in part using the inverse linescan model, featureedge positions of the identified feature; analyzing the feature edgepositions to determine errors in the manufacture of the patternstructure; and controlling a lithography tool based on the analysis ofthe feature edge positions.
 2. The method of claim 1, wherein thelithography tool is controlled by making an adjustment in at least oneof focus and exposure dose during the printing of a pattern structure.3. The method of claim 1, wherein the lithography tool is controlled toimprove the uniformity of feature edge positions from wafer to wafer. 4.The method of claim 1, wherein the lithography tool is controlled toimprove the spatial uniformity of the feature edge positions across awafer.
 5. The method of claim 1, wherein the lithography tool iscontrolled to improve the spatial uniformity of the feature width acrossa wafer.
 6. The method of claim 1, wherein the lithography tool iscontrolled to reduce the pattern placement error of the patternstructure.
 7. The method of claim 1, wherein the lithography tool iscontrolled to center the values of focus and exposure dose within thefocus-exposure process window.
 8. The method of claim 1, wherein thelithography tool is controlled to increase the size of thefocus-exposure process window.
 9. The method of claim 1, wherein thelithography tool is controlled to increase the throughput of thelithography tool.
 10. The method of claim 1, wherein the spatialdistribution or other characteristics of the light source of thelithography tool are adjusted to reduce errors in the feature edgepositions.
 11. The method of claim 1, wherein the lithography tool iscontrolled to improve the matching of feature edge positioncharacteristics to other lithography tools, such as spatial signaturesand pattern placement errors.
 12. An edge detection system, comprising:an imaging device configured to generate a first image of a patternstructure, wherein the first image of the pattern structure includesimage noise; and a processor, coupled to the imaging device, theprocessor configured to: generate, based at least in part on the firstimage of the pattern structure, measured linescan information describinga pattern structure of a feature; apply the measured linescaninformation to an inverse linescan model that relates measured linescaninformation to feature geometry information; identify, based at least inpart on the applying the measured linescan model to the inverse linescanmodel, feature geometry information that describes a feature that wouldproduce a linescan corresponding to the received measured linescaninformation; determine, at least in part using the inverse linescanmodel, feature edge positions of the identified feature; analyze thefeature edge positions to determine errors in the manufacture of thepattern structure; and control a lithography tool based on the analysisof the feature edge positions.
 13. The edge detection system of claim12, wherein the lithography tool is controlled by making an adjustmentin at least one of focus and exposure dose during the printing of apattern structure.
 14. The edge detection system of claim 12, whereinthe lithography tool is controlled to improve the uniformity of featureedge positions from wafer to wafer.
 15. The edge detection system ofclaim 12, wherein the lithography tool is controlled to improve thespatial uniformity of the feature edge positions across a wafer.
 16. Theedge detection system of claim 12, wherein the lithography tool iscontrolled to improve the spatial uniformity of the feature width acrossa wafer.
 17. The edge detection system of claim 12, wherein thelithography tool is controlled to reduce the pattern placement error ofthe pattern structure.
 18. The edge detection system of claim 12,wherein the lithography tool is controlled to center the values of focusand exposure dose within the focus-exposure process window.
 19. The edgedetection system of claim 12, wherein the lithography tool is controlledto improve the matching of feature edge position characteristics toother lithography tools, such as spatial signatures and patternplacement errors.
 20. A tangible, non-transitory computer-readablemedium storing instructions that, when executed, cause a processingdevice to: obtain a first image of a pattern structure, wherein thefirst image of the pattern structure includes image noise; generate,based at least in part on the first image of the pattern structure,measured linescan information describing a pattern structure of afeature; apply the measured linescan information to an inverse linescanmodel that relates measured linescan information to feature geometryinformation; identify, based at least in part on the applying themeasured linescan model to the inverse linescan model, feature geometryinformation that describes a feature that would produce a linescancorresponding to the received measured linescan information; determine,at least in part using the inverse linescan model, feature edgepositions of the identified feature; analyze the feature edge positionsto determine errors in the manufacture of the pattern structure; andcontrol a lithography tool based on the analysis of the feature edgepositions.